John Benton was a member of UNIT during the Doctor's exile on Earth, and after. He was promoted from Corporal to the rank of Warrant Officer Class 1, holding the post of Regimental Sergeant Major. One source says that he later achieved the rank of Lieutenant.
Profile
Early life
The father of John Benton served in the military. As a boy, John looked up to him and swore to "be just like him". (RP: Wartime) The Ninth Doctor and Rose Tyler met Benton's father in the trenches of World War I in 1914. (IDW: The Forgotten)
Military career
Benton first appeared alongside as a Corporal with the second incarnation of the Doctor against the Cybermen and Tobias Vaughn, though when he first met the Doctor he was doing surveillance work rather than serving in uniform. (DW: The Invasion)
By the time of the Doctor's regeneration and exile, Benton had been promoted to Sergeant and quickly formed a close working relationship with the Doctor, Captain Mike Yates and Brigadier Lethbridge-Stewart. Lethbridge-Stewart, in fact, usually called Benton rather than Yates when an order needed to be carried out.
Benton was always a down-to-earth example of a British infantryman and non-commissioned officer, reliable, uncomplicated and with a good dose of common sense. When the Doctor asked him if he was going to comment that his TARDIS was bigger on the inside than on the outside (since everyone did), Benton's classic reply was, "Well, that's pretty obvious, isn't it?" (DW: The Three Doctors) Despite not understanding the specifics of what the Doctor was doing or any of his explanations, Benton always took the Doctor at his word and trusted him implicitly. Benton was loyal, not just to the rank but also to the people he worked with, and was willing to disobey orders to help them (DW: Invasion of the Dinosaurs).
Little, though, is known of his personal life at this point, though it seems Benton was at that time single and preparing on a dancing date with his sister when a crisis with Azal interrupted his plans. (DW: The Dæmons) He flirted briefly with Jo Grant and Sarah Jane Smith, though as we far as we know this never progressed beyond the level of good-natured banter.
Benton had some memorable experiences during his time in UNIT. He fought against the Daleks. (DW: Day of the Daleks) The Master's TOMTIT machine had the effect of regressing Benton to a baby. When restored to adult form he appeared before his UNIT colleagues stark naked. (DW: The Time Monster) Benton also worked alongside both the current Doctor and the Doctor's previous incarnation, who the Time Lords had called in to help against Omega. Benton was delighted with the reunion and with the chance to work alongside him. (DW: The Three Doctors)
Shortly before the return of the Doctor to Earth and his regeneration into his fourth incarnation, Benton had his promotion to Warrant Officer. (DW: Robot). As a Regimental Sergeant Major, Benton helped against the Zygons. (DW: Terror of the Zygons)
Life after UNIT
In 1983, Brigadier Lethbridge-Stewart said to the Doctor that Benton had left the organization in 1979 and became a used car salesman. (DW: Mawdryn Undead) Sometime later he was married and had three children. (EDA: Genocide)
Benton celebrates his seventieth birthday on 6th January 2008, with a party attended by, amongst many others, the Sixth Doctor and Mel. Benton meets this incarnation of the Doctor for the first time on this occasion (Time's Champion) and plays a vital role in the multiversal catastrophe which ensues later.
By 2010, when he attended Bernice Summerfield and Jason Kane's wedding, he was in his early 70s. (NA: Happy Endings)
Apocryphal information
One account states that by 1986, Benton had returned to UNIT and became a commissioned officer with the rank of Lieutenant, subsequently leading a UNIT team to Antarctica to clear up the chaos left in the wake of the Cybermen's attempt to drain Earth of its energy. (DWN: The Power of the Daleks)
Behind the Scenes
Benton would have appeared in a cameo in The Five Doctors, not recognizing the Second Doctor and denying him admittance to UNIT's reunion, however John Levene declined to take up the role again on the grounds that Benton would have remembered that version of the Doctor and not made such a mistake. Instead, he was replaced by the character of Colonel Charles Crichton.
During his onscreen appearances, Benton's first name was never given. It was only revealed in Wartime.