In the original series, he is voiced by the late Ray Barrett, who also voiced the show's main antagonist, The Hood, as well as voicing Alan in the pilot episode "Trapped in the Sky".
The second son of Jeff Tracy (founder and financier of International Rescue), John was named after astronaut John Glenn. John is a noted scholar of astronomy and has authored several popular astronomy textbooks. He is a graduate of Harvard University with a degree in Advanced Telecommunications. John's quiet intellectual nature and interest in astronomy make him the natural choice for the solitary life as the occupant of space station Thunderbird 5, monitoring for distress calls from around the world. He has only ever been seen physically involved in a rescue during the episode "Danger At Ocean Deep", although he says at the end of that episode that he has already been on a "dozen or so" rescues. According to Carlton Books 'Lady Penelope's Secrets,' John is known to be exceedingly patient, kindly and gracious and possessed of both great intelligence and poise as gifts inherited from his talented mother.
Thunderbirds 2004 film
Lex Shrapnel portrayed John in the 2004 live action film. However, due to the film focusing on Alan Tracy, not much is known about this version of John, except for his age of 22, and the fact he's really fond of pizza, as revealed in the conversation scene between John and Jeff, before Thunderbird 5 is shot by the Hood. This also shows him to be the calm and reasonable one, a similarity with his original self, as John and Jeff discuss Alan's recent behaviour.
Thunderbirds Are Go!
In the 2015 series, John is still the 'pilot' of Thunderbird Five, but his role is greatly expanded beyond simply receiving calls for help, often providing aid in coordinating the rescue missions via long-distance communication. He is voiced by Thomas Brodie-Sangster (who also voiced Ferb Fletcher).
John is shown as being an emotional introvert, who prefers spending his spare time eating bagels, watching TV (his favourite show is Gerry Anderson's show Stingray, which featured his character Troy Tempest) and stargazing. He is generally a loner, who doesn't like having to spend more time on Earth than he has to, as he's now the lone operator of Thunderbird 5, until he creates an AI called EOS, which becomes his companion. He is also shown as being a redhead, as opposed to being a light blonde in the original series and live-action film.
Personality in Thunderbirds (1965-66)
John is the quietest and most intellectual of Jeff's sons, slighter in build than his elder brother Scott and younger brothers Virgil, Gordon and Alan. He is also tremendously lithe, sharp-minded, has a lot of mental energy, and is graceful. He sometimes gets frustrated with his duties aboard Thunderbird 5 because he is unable to take part in as many rescue missions as his brothers. While John may appear to act over-enthusiastically when dealing with emergencies, he is very quick at assessing a situation, proving invaluable when dealing with distress calls.
Personality in TAG
John is a technical whiz who is perhaps the most serious and sensible of the brothers, sometimes to the point where jokes fly right over his head. He is strong willed, quick thinking, and he almost always gets an important role in rescue missions by scanning environments and hacking networks and machinery. He is incredibly calm and he never raises his voice, even when speaking to people who are in grave danger and blind panic below, whether they be strangers or his own brothers. He loves stargazing, watching television, talking to himself, and eating yummy bagels in his spare time, and he also loves to work with, and talk to, his brothers, although Alan can annoy him sometimes as seen in Runaway. One thing he also loves, however, is his alone time.
He claims that the Earth "feels like the right distance to me" from Thunderbird 5 in Runaway, and he actually struggles to cope with being on Earth for too long as seen in Skyhook. Despite this, there is also something about his loneliness that he hates as well; in the episode EOS, John had the option to deactivate EOS with the slam of a big red button before him, but instead decided he would rather risk his life trying to befriend the rogue A.I. instead of pressing it because, as he put it: "I want a partner... I want a friend!".