For many years, Sewell was the gritty face of crime and law enforcement in a huge array of television series. Amongst his early roles, he was the tallyman in the television play Up The Junction (1965), a Criminal who runs off with a teenage girl in Softly, Softly (1966), a hard-nosed building Engineer in The Power Game (1965–66), a cowardly informer in Man in a Suitcase (1967), and a seedy private eye in Spindoe (1968). In 1969 he played an escaped convict called Jansen in the Randall and Hopkirk (Deceased) episode called "Vendetta for a dead man". In 1970, he played Colonel Alec Freeman in Gerry Anderson's live-action science-fiction drama UFO. Also in 1970 he played resistance leader Pierre Allard in episodes 14, [One Way Home] 17,{The Ugly Side of War] and 22,{Intent to Steal} of the hit (ITV series) Manhunt which was filmed in 1969 and aired on ITV in January 1970.