Sentinel Major was originally one of the Quintessons' slave robots on Cybertron around twelve million years ago. Unusual among his consumer goods brethren, Sentinel Major was equipped for combat and was forced to participate in gladiatorial games for the Quintessons' amusement, wielding a spiked ball-and-chain weapon and a tire-shaped shield. He was once defeated in battle by Prima, who attacked their Quintesson masters when their match was over, only to be destroyed himself.
Subsequently, the consumer goods robots rose up against the Quintessons in a rebellion that ultimately forced the aliens off the planet. Sentinel Major survived this clash, but he and the rest of his race—now dubbed "Autobots"—were robbed of any chance of peace when the military hardware robots, now known as "Decepticons", set their sights on conquering Cybertron for themselves. Two Autobot leaders fell during the war that ensued, and Sentinel Major inherited the Matrix of Leadership and became Sentinel Prime, leader of the Autobots. Under his leadership, the Autobots realized that they would never be able to defeat the Decepticons with firepower, and they turned instead to stealth, reconfiguring their bodies to be able to transform into other forms. With the power of his new ability, Sentinel Prime defeated the Decepticons' leader, and the Autobots won the war.
The ensuing period of peace was known as the Golden Age of Cybertron, but, around nine million years BC, the Decepticon threat re-emerged when they too adopted transformation, and coupled it with robot-mode flight powers. One of the first of this new breed of Decepticons, the malevolent Megatron, killed Sentinel Prime in combat. As he died, Sentinel Prime handed off the Matrix to Alpha Trion, who kept it safe until the emergence of the next Autobot leader, Optimus Prime. When Rodimus Prime journeyed into the Matrix in 2005, Sentinel Prime was the sixth guide he encountered. Sentinel Prime recounted the development of transformation, the Autobot victory in the war and the rise of Megatron.
Sentinel Prime was said to be the successor of Prime Nova and the predecessor of Optimus Prime y the Creation Matrix in Dark Creation. As seen in flashback in The Legacy of Unicron, the mortally wounded Sentinel passed the Matrix onto Optimus.
In Hardwired, Sentinel Prime was Autobot leader for the first 500,000 years of the Great War. He was armed with trithyllium alloy armor, an electro-disrupter and a large variety of other weapons, including various forms of artillery shell, a solar energy rifle and energon-charged sword.
8.2 million years ago, as mentioned in The War Within, Sentinel Prime faced Megatron on the battlefields of Tyrest. He proved no match for the Decepticon leader; with his last words, he tried in vain to reason with Megatron, but Megatron was unmoved by his "sermon and doctrine" and slew him. Sentinel, aware of his possible death, had already removed the Matrix of Leadership from his body before entering battle, preventing Megatron from acquiring it.
In Hardwired, during the war on Earth, the Matrix used a vision of Sentinel Prime to strengthen Optimus Prime's resolve during a battle with Megatron by confronting him with the claim that Megatron had chosen Optimus to be Prime, knowing that he was weak. Soon after, the Keepers created a robotic duplicate of Sentinel Prime to confuse Optimus and undermine his confidence. Though possessing all of Sentinel Prime's considerable abilities, Optimus eventually realized that the duplicate had been created from his own memories of Sentinel Prime, and was incapable of doing anything new. Optimus destroyed the duplicate of Sentinel Prime, having largely overcome his own doubts about his fitness to be Autobot leader.
- Main article: Sentinel Zeta Prime