Clemens was born in Croydon, Surrey to Suzanna (née O'Grady) and Albert, an engineer, who also worked in music halls. Clemens left school aged 14.
Following National Service in the British Army at Aldershot, where he was a weapons training instructor in the Royal Army Ordnance Corps, Clemens wanted to be a journalist but decided he did not have any qualifications. He was offered a job with a private detective agency, but this involved taking a training course in the Northern English 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. Instead, he worked his way up from messenger boy at the J. Walter Thompson advertising agency. While he was a copywriter there, he had a thriller screenplay accepted and shot by BBC TV - Valid for Single Journey Only (1955). This brought him to the attention of independent, low-budget movie producers, the Danziger brothers.
From the mid-1950s onwards, Clemens was a staff writer for the Danzigers, churning out dozens of quickie scripts for assembly-line 'B' movies and half-hour television series such as Mark Saber (ITV, 1957–1959; aka Saber of London), White Hunter (ITV, 1958–1960), The Man from Interpol (ITV, 1960–1961), and Richard The Lionheart (ITV, 1961–1965).
He also wrote for ITC Entertainment's thriller series The Invisible Man (ITV, 1958–1959), Sir Francis Drake (ITV, 1961–1962), and Danger Man (ITV, 1960–1961; 1964–1967; aka Secret Agent), for which he had also written the pilot. His output was so prolific during the late 50s and throughout the 1960s that he frequently used the pseudonym Tony O'Grady.
He wrote the original pilot episode for The Avengers in 1961 and was the script editor, associate producer and main scriptwriter for The Avengers series (ITV, 1961–1969) and, according to the British Film Institute's profile of him, "brought this spirit of burlesque to his other series - most notably with Adam Adamant Lives! (BBC, 1966-1967), but also with The Baron (ITV, 1966-1967), The Persuaders! (ITV, 1971-1972), The Protectors (ITV, 1972-1974), and The Adventurer (ITV, 1972-1974) - resoundingly poking fun both at the genre they were imitating and the sources of their inspiration."
It was Clemens who cast Diana Rigg to replace departing star Honor Blackman in The Avengers. He was later quoted as saying, "I didn't do Diana a very good service. It made her an international star but I think I could have done more for her as far as the script was concerned. She was rather a stooge to Patrick Macnee's Steed." He did not choose Linda Thorson to replace Rigg.
Clemens created the BBC TV sitcom, My Wife Next Door (1972) but left the scriptwriting to Richard Waring. The series won a BAFTA Award as Best Situation Comedy Series. Made around the same time, the TV movie The Woman Hunter (also 1972) was scripted by Clemens and fellow ITC writer Tony Williamson from the former's story. It was Clemens' first American credit.
He followed this with a twist-in-the-tail anthology series Thriller (ITV 1973-1976; aka Menace), for which he wrote all the stories as well as 38 of the scripts.
Clemens company The Avengers (Film and TV) Enterprises Ltd created as a French/Canadian/British co-production The New Avengers (ITV 1976-1977). The series cost £125,000 an episode to produce and was not a critical success, but sold to 120 countries. To cast the central female role of Purdey, Clemens considered "about 700 girls", interviewed 200, read scripts with 40 and screen-tested 15 before choosing Joanna Lumley. His company Avengers Mark One Productions went on to produce The Professionals (ITV, 1977–1983).
In the early 1980s, he was twice asked to produce a United States version of his most successful series - The Avengers U.S.A. for producer Quinn Martin and The Avengers International for Taft Entertainment but neither version materialised. 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).
Back in the UK, he worked on the BBC TV's Bergerac (1981–1991), the anthologies Hammer House of Mystery and Suspense (ITV, 1984–1986) and Worlds Beyond (ITV, 1984–1989), and adapted Gavin Lyall's espionage thriller The Secret Servant as a 3-part drama for BBC TV (1984).
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 also wrote three teleplays. He also wrote for the Dick Van Dyke mystery series Diagnosis: Murder (CBS, 1992–2001).
He also wrote for the Bugs TV series in the UK (BBC, 1995–1999) and Highlander: The Series in the US. Clemens' final credit was for Jane Doe: How To Fire Your Boss in 2007.
In 1971 he wrote and produced for Hammer films Dr. Jekyll and Sister Hyde and, in 1974, wrote and directed Captain Kronos, Vampire Hunter (his only directorial effort). 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 (1974), The Watcher in the Woods (1980), and Highlander II: The Quickening (1991).
In 2008 Clemens wrote the play Murder Hunt, which was performed at The Mill at Sonning and starred David Monteith as Captain K'Maka, a native African policeman who has to find the murderer amongst a bunch of guests stranded at a remote safari lodge.
In a British High Court of Justice case in the mid-1970s, which was abandoned by both sides due to escalating costs, Clemens claimed that he had told writer Terry Nation the concept for Nation's 1975 TV series Survivors in the late 1960s and had registered the idea with the Writers' Guild of Great Britain in 1965. Nation strenuously denied this.
Clemens married his first wife Brenda Prior in 1955 and were divorced in 1966. From 1967, he was with the actress Diane Enright, who was Diana Rigg's stand-in as Emma Peel during the 1965-1967 Avengers series. Enright committed suicide in 1976. He then married Janet Elizabeth with whom he had 2 sons; they stayed together until his death.
Clemens was appointed Officer of the Order of the British Empire (OBE) in the 2010 Birthday Honours. Clemens died on 10 January 2015, aged 83.At the Stroke of Nine (1957)
Woman's Temptation (1959)
Identity Unknown (1960)
Station Six-Sahara (1962)
Highlander II: The Quickening (1991)