As a member of Oliver's team. John is his partner and plays a number of roles including field support, decoy and guidance to Oliver in times of doubt. John has a deep hatred for Floyd Lawton for killing his brother Andy nd is also on a mission to find him and kill him. John was a temporary member of the Suicide Squad.
Before he was hired to protect Oliver Queen, he was in the army. While with his unit in Afghanistan, the group ended up in a trap. The men of the unit were all killed, but John killed the entire group of assailants. After one of his times back home Diggle saw the "ghost" of a fellow soldier who had died, a symptom of survivor's guilt, and only made it go away once he figured out what the ghost was trying to tell him. While in Afghanistan, he married fellow soldier Lyla Michaels. Their marriage fell apart sometime after they returned to the States, and John went back to Afghanistan for his third tour of duty. During his time in Afghanistan Diggle also met Ted Gaynor, a fellow soldier who saved his life from the enemy.
In "Pilot", he was hired by Moira Queen to protect Oliver. However the very first time Diggle drove Oliver somewhere he vanished from the car much to Diggle's annoyance. Later, Diggle went with Oliver again to his coming home party and guarded him. However when Oliver tried to leave Diggle tried to stop him but was knocked out, unknown to him that Oliver was actually heading next door as the new hooded vigilante to stop Adam Hunt.
In "Honor Thy Father", Oliver continued to try and avoid him.
In "Lone Gunmen", It is revealed that he had a brother, Andy, who died working as a bodyguard and thus left behind his wife. He discovers Oliver is Arrow after he is shot by Deadshot, and is taken to Oliver's hideout to recuperate.
In "An Innocent Man", After discovering Oliver is the Vigilante, he quit his job as Oliver's bodyguard stating how he did not approve how "Oliver spent his nights". Oliver later went to Diggle's sister-in-law's restaurant where Oliver tried to change his mind about his offer, and also revealed to him that Deadshot was Andy's killer which made John begin to rethink his role as a bodyguard. Later Diggle showed up to Oliver's house and accepted his offer, just in time to see Detective Lance arrest him.
In "Damaged", after Oliver was put under house arrest for being accused of being the vigilante, which was actually true, Diggle was told about a crime lord that was in trading arms. Diggle thought Oliver intended to head out himself but Oliver told Diggle himself to observe his closet men. Later Oliver had Diggle go out himself donned the Vigilante costume in order to save Oliver from incarceration.
In "Muse of Fire",
In "Year's End", after Oliver fails in taking down the Dark Archer, Diggle drives Oliver to a hospital and tells everyone he was hit by a semi while riding his motorbike.
In "Trust but Verify", John tried to convince Oliver that his former commanding officer, Ted Gaynor, was a good man. Unfortunately, he was wrong, and it nearly cost him Carly's life.
In "The Odyssey", Diggle is surprised to see Felicity Smoak come to the Arrowcave but discovers Oliver was shot. He saves Oliver's life with blood that Oliver stored away for a "Rainy Day" which Diggle referred to as it "Pouring" as Oliver was near death. He later used defibrillators to resuscitate Oliver when he went into cardiac arrest. Afterwards he was talking to Felicity, about him and Oliver, and she commented on how bad all their lies had been and Diggle told her that it was thanks to her, that they'd been able to do half of their work.
In "Dead to Rights", John was training Felicity, wanting her to be capable of handling herself. Then Oliver returned and John asked how the earlier mission went, and Oliver answered "Bad for him". Oliver then informed him and Felicity of that it was an assassin named, Guillermo Barrera, and he needed Felicity to hack his phone in order to figure out who his target was. When Felicity hacked Barrera's phone she was able to see that the last number that was called was to a restaurant called Jade Dragon, and John revealed that it is a front for the Chinese Mafia. Then when Felicity had hacked the phone they saw that the target is Malcolm Merlyn. He interrupts Oliver and McKenna Hall's date and informs Oliver of who the target is. After Oliver got back from saving Malcolm, Oliver informed John that Malcolm was taken to the hospital stating that it was good he wore a bulletproof vest. Oliver tells him that Malcolm is not at the hospital for being shot, he is there because of being poisoned by curare. John then realized that Floyd Lawton, his brother's killer, is still alive.
In "The Huntress Returns", John was watching the Starling City Sentinels report of Deadshot's kill until Oliver arrived and John showed him a video of Helena Bertinelli killing Gus Sabatoni, and he warned Oliver of that she might tell someone about them and their operation. Oliver told John he wants him to get in touch with their contacts in the Bratva and ask anyone in the street if they know why Helena is back and what she want. Later Oliver tells him that Helena paid a visit to his house and made a treat and he want security on his mother and sister. John tells him that he is way ahead of him. Later when Helena had blackmailed Oliver into helping her kill her father, John talked to him about Helena thinking Oliver should kill her, the Oliver asked him what he would do if it was Floyd Lawton and he answered not what Helena is doing. Oliver called him and when he came to Felicity's office there he found Oliver freeing a tied up Felicity.
In "Salvation", Oliver tried to take down John Nickel, a corrupt businessman but when Oliver arrived he was already taken by someone else. After Oliver attempted to go after someone else on the list Dig took the list from him and insisted that he give it a rest and the two went to Big Belly Burger. Dig expressed his concern about how Oliver is handling the drama in his life whilst being the Hood such as his flings with McKenna and Helena but everyone in the bar got a video streamed onto the phones of a man named "Savior", real nameJoseph Falk, had Nickel strung up and killed him. Later Joseph did the same again to Gavin Carnaham as Oliver tried to find him with Felicity's assistance but was too late. Later while trying to find a way to narrow Joseph down Dig noted the train moving sounds in his broadcasts and realised that they were in a subway car which lead to Oliver saving Roy Harper from him.
In "Unfinished Business",
In "Home Invasion", John and Oliver planned to take down Deadshot. With Felicity hacking A.R.G.U.S's database, they learned that Floyd was returning to Starling City to met a potential client, which really was A.R.G.U.S setting a trap for Deadshot. When John went to the mall, the trap was already taking place. He observed the A.R.G.U.S agents (including Lyla) while waiting for Floyd to show up and when Floyd was started sniping the agents John quickly ran towards Lyla, and was able to save her and then he ran to stop Floyd from escaping. John ran into Floyd in the staircase, the two fought, and Floyd got hold of John's gun. Floyd held John's gun at his face, and he forced him to fall on his knees with his hands behind his head. John told Floyd "I'm gonna kill you for what you did to Andy". Floyd answered, "I would pull the trigger right now, but no one is paying me" and he told John, "I got space for you right here next to your brother", showing John his victim tattoo of Andy. Floyd then punched him in the face with his gun, and left John on the floor. Felicity later tended to his wound and when Oliver came back, John was angry at him for not showing up when he needed him. When Oliver later tried to get on the same page as John, John told him they were done with each other and quit being a member of the team.
In "The Undertaking", Felicity goes to John, and tells him about the new lead. He says he will come back as soon as Oliver apologizes, but because of his stubbornness to admitting his wrong, he knows Oliver won't. After overhearing Moira and Malcolm talking, Oliver realizes that he will need all the help that he could get. Oliver apologizes to John, and told him about Moira's involvement in The Undertaking, saying he needed John's help to stop it.
In "Darkness on the Edge of Town", John is back on the team. He pretends to be the Vigilante to get information from Moira, hitting Oliver to gain information. He was meant to draw back his punches, but Oliver comes back limping and injured. Later Dig assists in getting Oliver and Felicity into Merlyn Global to find out Malcolm's plan with Dig posing as a security guard and even helping Felicity out of trouble when she is almost compromised. Later Oliver confessed that once Malcolm is dead that he will stop being the Hood to a surprised John and Felicity. Later when Felicity finds the device Oliver sends John to disarm it while he handles Merlyn. However the device is gone and Oliver is overpowered and captured by Malcolm.
In "Sacrifice", Diggle saves Oliver from custody of Malcolm Merlyn and the two return to the Arrowcave with Felicity. Later Oliver, Diggle and Felicity analysed the date for Merlyn's timetable and discovered the device to be in the railway tunnel under the city and Diggle was present as Moria confessed the truth on the news. Oliver initially tasked Diggle to go after the device but he convinced Oliver to let him go with against Malcolm together while Detective Lance handles the device with Felicity's tech support. They briefly engage the Dark Archer, but John gets injured and has to let Oliver go without him. He manages to get up the stairs in time to see Oliver kill Malcolm. Before dying however Malcolm revealed that he had a second device which forces Oliver to leaves John as he realizes that the earthquake was hitting Laurel's workplace.
In "City of Heroes", John and Felicity travel by plane to Lian Yu, to convince Oliver to return home. Oliver initially refuses, but he and Felicity convince him not to return as the vigilante, but as Oliver Queen, because he's needed more than ever as his mother's trial is coming up and a takeover of Queen Consolidated is imminent. Later, Oliver finds out that John lied to him, and only wanted him back to catch the group calling themselves, The Hoods. In a board meeting, The Hoods come after Oliver Queen, trying to kill him, but John intervenes. John and Felicity are later left wondering why Oliver avoided attacking The Hoods. Oliver tells them that they aren't considering the body count, Felicity asks him why he cares now, and John realizes it was "since Tommy". Later Oliver, Felicity, and John go to the upgraded HQ to plan to take down The Hoods, who have now captured Thea.
In "Identity", John and Oliver visit the Glades Memorial Hospital, and he tells Oliver how people from the earthquake ended up there. John and Oliver see Sebastian Blood talking to press outside as they leave the hospital. John tells Oliver that Sebastian has very strong opinions and that he believes the Glades have been abandoned, and that no one gives a damn about the people in the Glades. After a quick chat between Oliver and Sebastian, they leave, with some of the protesters becoming violent. Later, John and Oliver, as The Hood, take on China White and Bronze Tiger. John drives away in one of the trucks, but is soon after attacked by China White, and they face off. As China White gains the upper hand, Oliver shoots a trick arrow at her, pinning her to a nearby pole.
In "Broken Dolls",
In "League of Assassins",
In "Keep Your Enemies Closer", it is revealed that Lyla Michaels and John met in Afghanistan, and then got married. At the end of John's second tour, they rotated back Stateside, where their marriage could not survive the peace, and they divorced. John went back to Afghanistan for a third tour, while Lyla joined A.R.G.U.S. In the episode, Lyla is missing in Russia, having been captured during an intel operation in a Russian gulag. John is recruited to break her out, because A.R.G.U.S. does not have sufficient resources for the operation, and her superiors already consider her disavowed and dead. Oliver and Anatoly arrange to get John arrested and sent to the prison, where he finds Deadshot, whom he teams up with to break Lyla out of jail. Afterwards, John chooses to spare Deadshot's life rather than kill him in cold blood. In return, he is informed that an organization called H.I.V.E. hired Deadshot to kill his brother Andy. After returning to the States, John and Lyla rekindle their relationship.
In "State v. Queen", John collapses after being unknowingly dosed with Vertigo. He is taken to the Arrowcave to recover. While recuperating, he and Felicity deduce that Count Vertigo is distributing the drug to the citizenry of Starling City through flu vaccination trucks.
In "The Scientist",
In "Three Ghosts", John goes on a recon mission to gather information about Cyrus Gold. Cyrus finds him and overpowers him in a brief confrontation, but John manages to escape.
In "Blast Radius",
In "Blind Spot",
In "Tremors", When Oliver revealed his identity to Roy, he remained skeptical about Oliver's choice. However he later welcomes Roy onto the team.
In "Heir to the Demon",
In "Time of Death", While Felicity was growing jealous of Sara, Diggle comforted her with little success. Later he went out with the rest of Team Arrow to stop William Tockman. After being told that the pressure valves would destroy three city blocks, he managed to cool them down just in time.
In "The Promise", Diggle and the rest of Team Arrow went to take down Slade at the Queen Mansion. After Oliver got him outside, he was knocked unconscious by an unknown accomplice before he could take the shot. Slade later assures Oliver that he is alive for now.
In "Suicide Squad", John and Lyla are recruited by Amanda Waller to lead the Suicide Squad on a mission to destroy a nerve gas in the possession of Gholem Qadeer. John argues with Lyla about Waller's methods when the A.R.G.U.S. director kills Shrapnel for attempting to flee the mission. He is disgusted by Waller's callous disregard for the lives of her agents, while Lyla supports her boss. He manages to get an invitation to a party at Qadeer's mansion, which Deadshot uses to infiltrate the premises and locate the nerve gas. When A.R.G.U.S. sends a drone to destroy the mansion, John persuades Deadshot not to sacrifice himself for the mission. He persuades the guests to leave by denouncing Qadeer in front of them, and he, Lyla, and the squad flee. He later reconciles with Lyla, saying that she is more important to him than sticking rigidly to his principles. In a flashback sequence, John is seen serving as a soldier in Afghanistan. He helps capture Gholem Qadeer and saves his life when their group is attacked. He is also seen meeting Lyla for the first time.
In "Birds of Prey",
In "The Man Under the Hood", John along with Sara, Oliver and Felicity broke into Queen Consolidated's Applied Sciences Division. Working to get everyone in outside before they set off the bombs. When Oliver went to kill Slade Wilson Diggle backed him up by shooting Isabel before she can kill Oliver.
In "Seeing Red",
In "City of Blood", Diggle and Felicity attended Moira Queens funeral. Afterwards they continued to search for Oliver. They were forced to go to Amanda Waller for help. Later he along with Oliver, Felicity and Laurel went to cripple Slade Wilson's army. He was about to set of the charges before Isabel Rochev stopped him and threatened to kill him.
In "Streets of Fire",
Return of the Church of Blood
After The Arrow and Roy grappled on to a drug plane, Felicity attempted to hack the plane's Wi-Fi, though when she couldn't get a clean signal, Diggle suggested "plan B", namely invading the plane and placing an auto-pilot device in the cockpit. However, while they were doing so, Roy was shot by one of the men they had not knocked out. As he fell from the plane, Oliver suggested Dig prep for surgery. When they returned to the foundry they performed the surgery. Deciding that all they could do was wait, Dig left to tend to a pregnant Lyla.
Sometime later, Dig went to visit Lodai, their supplier. He commented on Lodai's paranoid guards, and promised to bring more money for the goods "next time", a claim that Lodai had heard numerous times. He called Oliver to update them, mentioning how their transactions were much easier when Oliver was a billionaire.
Following her run-in with a member of the Church of Blood, Laurel returned to the base to tell the team of her news regarding a resurfaced Brother Blood. Despite the original Brother Blood, Sebastian Blood, being dead, Diggle suggested Oliver go check out St. Walker's Church, an abandoned orphanage that Sebastian had been raised in. After Oliver didn't check in, Dig went to provide backup. He attempted to enter the church, but was distracted by a follower, only for another to come up behind and knock him out, identifying Diggle as one of the two people Brother Blood had been looking for.
Diggle assisted The Arrow and Roy in the field as they captured Vincent Stilgrave. Later he got into a heated argument with Oliver, who forbid Diggle from putting his life in danger as he's about to become a father. When Oliver visited Diggle and Lyla in the hospital, Diggle admitted that Oliver was right that he should be with his daughter.
Despite being told to be with his family, Diggle told Oliver he will help go after the man who killed Sara. Diggle had the belief it was a member of the League of Assassins who killed Sara but was told they'd never go after their own kind. Given access by Lyla, Diggle used A.R.G.U.S. in search of possible suspects for Sara's murder, to which he found Simon Lacroix, a mercenary archer who wears a full face mask. At Sara's burial, Diggle informed Laurel that he and Lyla decided too name their baby Sara, to which Laurel was thankful to hear. After hearing Simon Lacroix wasn't Sara's murderer, Diggle told Oliver he'll stay until the killer is caught.
Lyla asked Diggle to follow Oliver to Corto Maltese and check up on an agent, Mark Shaw. Diggle made contact with Shaw and verified his identity with a security key that contained the identities of all A.R.G.U.S. operatives and their families, only for Shaw to double-cross him and steal the key in order to sell it. Diggle and Oliver inform Lyla and she teold them to wait while she sends an A.R.G.U.S. tactical team to handle the situation. However, they ignored her and attacked Shaw as he met with his buyers. Enraged, Diggle chased down Shaw, beat him, and demanded to know what gave Shaw the right to endanger his family by selling the key. Shaw replied that it was his only escape from A.R.G.U.S. and Amanda Waller, who he felt had taken over his life. Later, when Diggle returned to his family, Lyla said she had to go to A.R.G.U.S. to report to Waller, and Diggle looked briefly troubled.
After Nyssa returned and announced Malcolm is still alive, Diggle asked what her source was. Later Diggle along with Nyssa and the team went to Ken Zhi Jansen's temple though Diggle only found his grave. After Oliver tagged Malcolm with a tracker, he was told to track Malcolm and was told that even an idiot could run it. Later, Oliver had Diggle secure the perimeter as Oliver couldn't risk Thea seeing him.
When Brother Eye shut down the power in Starling City, John arrived by Verdant to go down the arrowcave though he had to bring his child as Lyla was out on a mission and the nanny called in sick. Oliver told Diggle he wasn't comfortable with a baby down in the cave. John left his child in the care of Donna Smoak. When Oliver was worried Felicity was taking too long, Diggle told Oliver that an A.R.G.U.S. agent dropped by not to long ago to pick up his child and that he'd tell him to go back and check on them. Diggle later arrived to stop Brother Eye from robbing the truck of cash.
Diggle, Oliver, and Roy looked for a drug dealer under the alias of "Pakko", where they found the word "Guilty" written with blood. Diggle later believed that Ted Grant was the culprit despite Laurel vouching for him. After Roy revealed his nightmares which he belueved was true, Diggle stopped him when he tried to leave though Oliver told Diggle to let him go. Diggle later told Oliver that he should cut Roy loose as he believes he isn't fit to be on the team, believing he really did murder Sara. Diggle and Oliver later went to rescue Laurel and Ted, who were kidnapped by Isaac Stanzler. Diggle later pulled Ted out of the car, who went unconscious briefly went Laurel lost control of the car.
He later helped Oliver as Felicity was gone and a mysterious archer had killed Isaac Stanzler with a arrow with a spade shaped point. Oliver was frustrated with Felicity hooking up with Palmer. John talked with him about it. Oliver discovered a small letter in the arrow that had had killed Isaac Stanzler. An address of a apartment was in it. The two of them broke in the apartment and searched it. Surprised they noticed there were articles of newspapers on the walls about the arrow. There was even a shrine where Arrows of Oliver were displayed. Without their knowledge Carrie watched them from a nearby building. Oliver realized the spade arrow was actually a heart. Carrie called them with a cell phone that was laying on the table. She told Oliver she wanted to be with him and then send him a picture of a mob boss with a bomb vest she wanted to kill. John stayed at Verdant as Roy and Oliver went to Sherwood to confront Carrie and free the mob boss. John tried to comfort Oliver and invited them to diner with him and Lyla. John later found out Carrie had killed a hacker and learned that he had made a algorithm for her that traced the Arrow his activities back to Verdant. He quickly informed Oliver after which he looked for Thea to protect her. Oliver went back and called her and managed to agree a meeting with her and let her leave the club. John and Felicity were following Oliver through his earpiece as he went to the place he first saved her. Diggle and Felicity were no longer able to follow Oliver his signal as he was down below. John was relieved learning Oliver had survived and he later told her Carrie had joined the Suicide Squad. John invited Oliver for a late dinner and was joined by Oliver and Roy just as they begun.
Contrary to his seemingly tough exterior, John is a very perceptive and wise individual, being the first to notice that something was different about Oliver after he came back from the Lian Yu. This was what prompted Oliver to have him as his comrade. Diggle hated personality security because he was protecting spoiled rich kids instead of making a difference in the military. This was what made him want to join Oliver's team after a conversation with Carly Diggle made him realize Oliver was making a difference and was offering him a chance to do the same.
Since becoming a member of Oliver's team, John has provided Oliver with a bigger sense of direction, such as when he suggests saving and helping other people from danger instead of hunting down every last men in his father's book. John also seems to fully be aware of his place. When Oliver agreed to help catch the Royal Flush Gang, something John barely convinced him to do, he didn't argue against Oliver breaking into the police station to gather the necessary information despite his obvious discomfort with the idea when he plainly stated "Please tell me you're not going where I think you're going" and left it at that. He also provides Oliver with wise counsel, even if Oliver doesn't agree with him sometimes. He is also seen to be very good friends with Felicity Smoak and the two seem to have a sibling-like relationship.
Due to his time in the military, John is a disciplined and level-headed person who tries to approach everything with a calm demeanor, and he is calm in times of crisis. Diggle also has the ability to isolate his emotions from his ability to think logically. Despite the fact Floyd Lawton killed his brother, he refused to let that get in the way of his principles of never leaving a man behind when they were partnered together. He also tried to talk Lawton out of his death wish while on a suicide mission from the Suicide Squad by bringing up his daughter Zoey, and the chance to see her again one day, succeeding in doing so. He also later confessed Lawton, the man who killed his brother showed much more character than Amanda Waller, the woman supposedly protecting the United States did during that mission. He has a sense of justice and honor, and is extremely loyal to the people he cares about. He also has a subtle and dry sense of humor.
- Peak of human physical condition: John engages regularly in intense exercises and spars with Oliver during his spare time.
- Extensive military training: John is well-trained in the armed forces disciplines, and has mentioned that he was once a part of Special Forces. He has considerable experience in live combat, having done three tours in Afghanistan.
- Expert hand-to-hand combatant: John is a skilled fighter, having developed his skills during his time as a soldier. He is skilled enough to match China White in combat and keep up with Oliver during their sparring sessions. John was also able to hold his own against the highly skilled Malcolm Merlyn, though he was supported by Oliver. However, he lost his fight against Deadshot, partially because Deadshot pulled two pistols on him. He has been increasing his skill in fighting as he regularly trains with Oliver. He was able to hold out against Isabel Rochev, who not only was trained in fighting, but was enhanced by the Mirakuru.
- Weapons proficiency: John can handle most type of firearms, having been trained in the army. He is proficient enough in handling small caliber to high caliber firearms such as a grenade launcher. He is also capable of handling Oliver's bow, although not being as effective and efficient.
- Expert military operator: John is well-versed in intelligence gathering, decoding ciphers and other coded messages, infiltration, and espionage.
- Bilingualism: Based on what has been currently shown, John is capable of speaking English and Arabic fluently.
- Guns/Firearms: John primarily makes use of a pistol, wielding it quite effectively. He also is proficient in using assault rifles, sniper rifles, explosives and grenade launchers.
- Flick Knife: Along with his sidearm, John also carries a small flick knife in to combat.
- Telescopic Baton: John has also been shown to use a telescopic baton, specifically fighting Ravager.
- Skiing Mask: Although Diggle does missions with Oliver's team without a mask, he sometimes uses a skiing mask to conceal his identity.