Sherwin began his career in the theatre and worked as a junior set designer, scenic artist, scene shifter, stage manager and lighting designer. Sherwin established himself as an actor in theatre, films and television.
While still working as an actor, Sherwin also began work as a freelance writer, contributing scripts to series such as Crossroads and Z-Cars. In 1967, Sherwin was offered a story-editing role on Doctor Who by BBC Head of Serials Shaun Sutton. He was script editor on the stories from The Web of Fear to The Mind Robber, and on the latter tale he wrote the first episode. He also wrote the script for The Invasion, which introduced the United Nations Intelligence Taskforce, having adapted the original storyline supplied by Kit Pedler.
In 1968/1969 Sherwin began to take a greater role in the producing side of the series and was the unofficial Assistant Producer for the rest of the series' sixth season, with Terrance Dicks succeeding him as script-editor. On The Space Pirates Sherwin briefly resumed his old role as Dicks was busy writing The War Games. Dicks has credited Sherwin with the creation of the Time Lords, who were introduced in The War Games, scripted by Dicks and Malcolm Hulke. Sherwin succeeded Peter Bryant as producer of the programme in 1969, overseeing the production of The War Games and Spearhead from Space. Sherwin was also involved with Bryant in the casting of Jon Pertwee in the lead role on the programme.
After Doctor Who, Sherwin was the producer on Paul Temple, The Man Outside, Ski-Boy, and The Perils of Pendragon.