Yates first appeared in the Third Doctor serial Terror of the Autons, the latest in a line of Captains assisting Brigadier Lethbridge-Stewart, although the Doctor implies that he was around for previous stories. He proved more durable than his predecessors, appearing semi-regularly in the programme from 1971 to 1974 alongside the Brigadier and Sergeant Benton. Together with the Doctor and his companions, Yates fought off alien invasions, the machinations of the renegade Time Lord known as the Master, rogue computers and mutated maggots.
Yates was a fairly non-descript character, typical of the British Army officer class, and little is known of him outside of his time with UNIT. He was cool under fire, efficient, and both gave and carried out his orders with a minimum of fuss. He appeared to have a relatively good rapport with his men, although he did remind Benton on occasion that "rank hath its privileges". He was attracted to the Third Doctor's assistant Jo Grant and they may have dated for a time - Jo was dressed up to go on a date with Mike at the start of The Curse of Peladon only to be taken on a trip in the TARDIS by the Doctor.
Yates's fall from grace started when he was brainwashed by the artificial intelligence BOSS in the 1973 serial The Green Death. The brush with ecological disaster apparently made Yates very concerned about the future of the planet, and he was easily recruited by Sir Charles Grover into a conspiracy to reverse time and return Earth to a "golden age". The conspiracy was thwarted by the Doctor, and in return for his past service to UNIT, the Brigadier allowed Yates to take medical leave and then quietly resign (actor Richard Franklin believes the initial plan for this story was to kill off Yates).
Trying to recover, Yates attended a meditation centre but uncovered strange goings-on, which he reported to Sarah Jane Smith, since he felt that UNIT would find him untrustworthy. Sarah communicated this back to the Doctor, leading into the rest of the events of the Third Doctor's last serial, Planet of the Spiders. It is not known if Yates returned to UNIT after that, or if he went on to other things.
Planet of the Spiders was Yates's last appearance in the series. Richard Franklin returned as an illusory image of Yates in the 20th Anniversary special The Five Doctors and reprised the role of Yates in the 1993 charity special Dimensions in Time.
As one of the more prominent recurring supporting characters in the television series, Yates is often listed as a companion of the Doctor and indeed is listed as such on the official BBC Doctor Who website. However, he is not always listed as such - the John Nathan-Turner penned book of companions, for instance, excludes Yates.