Clemens was born in Croydon, Surrey, to Suzanna (née O'Grady) and Albert,[3] an engineer, who worked in music halls.

He was offered a job with a private detective agency, but this involved taking a training course in the city of Leeds and, as he had been away from home in London for two years, he decided he did not want to go away again.

Beginning in the fourth season, Clemens became the script editor, associate producer and main scriptwriter for the series.

Due to his rapid working rate and productivity, Clemens would frequently perform extensive rewrites of other writers' scripts to ensure the show consistently matched his creative sensibilities; fellow Avengers writer Roger Marshall later noted that "his influence pervades almost every scene"[6] of the show's later seasons.

"[5] Clemens created the BBC TV sitcom My Wife Next Door (1972) but left the scriptwriting to Richard Waring.

Made around the same time, the TV movie The Woman Hunter was scripted by Clemens and fellow ITC writer Tony Williamson from the former's story.

[1] To cast the central female role of Purdey, Clemens considered "about 700 girls", interviewed 200, read scripts with 40 and screen-tested 15[1] before choosing Joanna Lumley.

[8] However, he did write episodes for the US TV series Darkroom (ABC-TV, 1981–1982), Remington Steele (NBC, 1982–1987), and Max Monroe: Loose Cannon (CBS, 1990).

[5] He then, in the US again, worked on the Father Dowling Mysteries (NBC, 1989; ABC-TV, 1990–1991), as executive script consultant for the feature-length revival series of Raymond Burr's Perry Mason (CBS, 1985–1995) for which he wrote three teleplays.

He also wrote the screenplays and/or stories for the feature films Operation Murder (1957), The Tell-Tale Heart (1960), Station Six-Sahara (1963), The Peking Medallion (1967), And Soon the Darkness (1970), See No Evil (1971), The Golden Voyage of Sinbad (1973), The Watcher in the Woods (1980), and Highlander II: The Quickening (1991).

The play has the great detective Sherlock Holmes and his colleague Dr. Watson become embroiled in the grisly murders in Whitechapel in 1888.

[11] The list of plays he helped to write and produce:[12] Clemens married his first wife Brenda Prior in 1955; they divorced in 1966.