Before becoming a comedy writer Renwick worked as a journalist on his home town newspaper, the Luton News. On beginning his comedy writing career in the mid-1970s he initially submitted material for BBC radio comedies including Week Ending and The News Huddlines. He also contributed to other radio series such as 'Oh, Get On With It!' starring Kenneth Williams and with David McKellar co-wrote 'Harry Worth in Things Could Be Worse' featuring Harry Worth. Teaming up with writing partner Andrew Marshall, they wrote the BBC Radio 4 comedy series The Burkiss Way and provided sketches for BBC television 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 Two Ronnies 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", adapted from his "Answering one question behind all the time" sketch from The Burkiss Way. 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 the sketch showAlexei Sayle's 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 into a feature film version. In 1983 they wrote The Steam Video Company for Thames Television, a short comedy series consisting of absurd parodies of famous novels. This was followed in 1986 by Hot Metal for LWT, a six-part satire of the tabloid newspaper industry starring Robert Hardy, Geoffrey Palmer and John Gordon Sinclair. The show was a critical success and returned for a further six episodes in 1988 with a revised cast of Robert Hardy, Richard Wilson and Caroline Milmoe. 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 remake titled Cosby, starring Bill Cosby, although this is generally regarded as a very loose 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 2016, thirty-two episodes have been produced across five short-run series and six specials. The slow rate of production is partly due to Renwick's writing of the episodes, which he describes as being a painstaking process in which the intricacies of the plots take several months to work out. 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. Renwick's fondness for rationalist murder mysteries with supernatural overtones, later developed fully in Jonathan Creek, is evident in elements he added to the Poirot adaptations. In 1992, Renwick and co-writer Michael Baker received an Edgar Award from the Mystery Writers of America for the Poirot episode "The Lost Mine", which aired in the US as part of the PBSanthology seriesMystery! 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 former writing partner Marshall, had cameo roles in an episode of the series as members of a television sitcom scriptwriting team.
