In addition to his producing role, Letts also directed several serials during his time in charge of the programme - "Terror of the Autons", "Carnival of Monsters" and "Planet of the Spiders".
In addition, he handled much of the direction for "Inferno" after Douglas Camfield was taken ill, and after leaving the series as producer he directed "The Android Invasion" for his successor, Philip Hinchcliffe. He also co-wrote "The Daemons" with Robert Sloman (under the pseudonym 'Guy Leopold') and worked heavily with Sloman on the writer's other scripts for the programme.
Letts's also created one of Doctor Who's most memoral characters: Sarah Jane Smith, played by Elisabeth Sladen. He was also deciding factor in the casting of Tom Baker as the Fourth Doctor.
Letts remained fond of and connected with the series right up until his death. When producer Graham Williams broke his leg during production of season 16 in 1978 Letts helped to keep an eye on the series, and more officially he served as Executive Producer in 1980, overseeing the inexperienced John Nathan-Turner's first season in charge of the programme.
For many years thereafter Letts wrote many novels, novelisations and radio serials connected to the series. He also appeared on many DVD commentaries and various documentaries. He also gained extensive credits outside of Doctor Who, most notably as producer of the BBC's "Classic Serial" strand during the late 1970s and early 1980s. In this role he produced many acclaimed and award-winning adaptations of classic novels, including "Great Expectations", "Alice in Wonderland" and "Jane Eyre". Later, he directed episodes for the soap opera "EastEnders".