Letts first work on Doctor Who was as director of the Patrick Troughton serial "The Enemy of the World", and he would direct four serials during his tenure as producer: "Inferno"note , "Terror Of The Autons", "Carnival Of Monsters" and "Planet Of The Spiders". After leaving as producer, Letts returned to direct the Fourth Doctor Serial "The Android Invasion".
Taking over as producer from "Doctor Who and the Silurians", Pertwee's second story as the Doctor, Letts inherited from his predecessor Derrick Sherwin a major change to show's format in which the Doctor was stranded on Earth, exiled by the Time Lords. Although neither Letts nor Dicks was a major fan of this limitation to the show's storytelling possibilities, they largely stuck to it for their first two seasons, before beginning to experiment with returning to space with their third, and completely lifting the exile with their fourth.
Letts was noted as an early pioneer of "Colour Separation Overlay", the forerunner of modern Chroma Key (Green Screen) technology. Its copious use in many stories of the Letts' era is very obvious to modern viewers watching the remastered DVD editions, and can come across as Special Effect Failure to modern eyes. However, it should be remembered that it would have been far less obvious and distracting to viewers watching on blurry television sets in the early 1970s.
Letts also co-wrote four serials during his tenure as producer with his friend Robert Sloman: "The Dæmons", "The Time Monster", "The Green Death" and "Planet Of The Spiders". BBC regulations precluded Letts from being credited as writer, since he was already credited as producer, so "The Dæmons" was credited to the pseudonym "Guy Leopold" and the remaining three gave sole credit to Sloman.
Letts departed as producer, along with Dicks and Pertwee after five seasons in the post, to be succeeded by Philip Hinchcliffe. One of Letts' final acts as producer was to cast a little-known, out of work actor called Tom Baker in the role of the Fourth Doctor. It would prove an inspired choice, and it would be seven years (and three producers) later before another new Doctor needed to be cast.