Brian Clemens left school at the age of 14. After national service with the Royal Army Ordnance Corps, he worked his way up from messenger boy to copywriter at an advertising agency, writing in his spare time. One of his scripts was accepted by the BBC in 1955. He joined a production company, literally writing scripts to order. With tight deadlines and plots often based on the availability of sets, props or location, he churned out scripts for B-films and TV series.

Clemens is best remembered for his work on British television in the 1960s and 1970s, especially on Danger Man (1960), The Avengers (1961) (for which he wrote many episodes, including the pilot in 1961), The Baron (1966), The Persuaders! (1971) and creating The Professionals (1977). He also wrote for the stage; his play "Strictly Murder" was performed by a cast including Brian Capron in 2017.

Clemens was awarded the OBE (Officer of the Order of the British Empire) in the 2010 Queen's Birthday Honours List for his services to Broadcasting and to Drama. According to his son Samuel, the last thing he did before he died was to watch an episode of The Avengers (1961) and his last words were: "I did quite a good job".