Before beginning his full-time comedy writing career, he worked as a journalist on his home town newspaper, the Luton News.

On beginning his comedy career, he initially worked in a team with writing partner Andrew Marshall, the pair of them providing material to popular sketch shows such as The Two Ronnies and Not the Nine O'Clock News during the late 1970s and early 80s. One of the most celebrated sketches he wrote for the former was a parody of the BBC quiz programme Mastermind, where a "Charlie Smithers" chose to answer questions on the specialist subject "Answering the question before last". They also wrote The Burkiss Way for BBC Radio 4, occasionally accompanied by other writers on early episodes. Their short-lived LWT series for ITV, End of Part One, was an attempt to transfer Burkiss-style humour to television. Later in the 1980s they also wrote for comedian Alexei Sayle's sketch show Stuff and Spike Milligan's There's a Lot of It About.

In 1982 they penned the comedy drama serial Whoops Apocalypse for LWT, based on the insanity of international politics in the age of nuclear weapons, and four years later they adapted the screenplay (changing most of the characters and situations completely) into a feature film version. In 1983 they wrote The Steam Video Company for Thames Television, a short comedy series based on very silly parodies of famous novels.

Renwick began writing solo in 1990 when he created the sitcom One Foot in the Grave, starring Richard Wilson, which was highly successful and went on to be a popular hit for the following decade. It also ran for four seasons as an American re-make entitled Cosby, starring Bill Cosby, although this is generally regarded as a poor adaptation of the original.

In 1997, Renwick devised the comedy-drama Jonathan Creek, based around the crime-solving abilities of the eponymous designer of magic tricks, played by comedian Alan Davies. As of 2004, twenty-six episodes have been produced across five short seasons and two Christmas specials. The slow rate of production is partly due to Renwick's writing of the episodes, which he describes as being a painstaking process taking him several months to establish all of the intricacies of the plots.

He has also written for 'straight' television drama, contributing episodes to ITV's famous adaptations of Agatha Christie's Hercule Poirot mysteries, starring David Suchet.

Most recently, another comedy-drama Renwick has penned, entitled Love Soup, starring Tamsin Greig and Michael Landes, premiered on BBC One on 27 September, 2005. Renwick, and his ex writing partner Marshall, had cameo roles in an episode of the series as members of a television sitcom scriptwriting team.