Benton was a member of UNIT when the Third Doctor worked alongside them when he was in exile on Earth.
Benton was apparently one of the founding members of UNIT. Corporal Benton was serving in the Army under Colonel Lethbridge-Stewart in "The Invasion", and when the Colonel was promoted to Brigadier and put in charge of the UK division of UNIT, he brought along Benton and promoted him to Sergeant. Benton was still there (as a Regimental Sergeant Major) in the Brig's last UNIT appearance, but apparently mustered out and become a car salesman around the same time the Brig retired (as mentioned in "Mawdryn Undead").
For story purposes, Benton usually served as the 'typical enlisted soldier', just as Mike Yates was the 'typical officer' and the Brig was the 'typical commander'. While he was a pretty major character during the UNIT era, his first name was never mentioned once.
He also had one of the best "bigger on the inside" reactions of any character seeing the inside of the TARDIS for the first time, in "The Three Doctors":
- Third Doctor: "Well, sergeant, aren't you going to say that it's bigger on the inside than it is on the outside? Everybody else does."
- Benton: "It's pretty obvious, isn't it?"
- ↑ The novels named him John, which originally came from the non-licensed UNIT videos from BBV.