Olsen was born Gary Kenneth Grant in London and lived with an aunt and uncle after losing both his parents, Patricia and Kenny, at an early age. He attended the Archbishop Tenison’s Church of England School in Kennington. After school he joined various junior stage groups and toured with fringe theatrical companies, such as Incubus and Lumiere and Son until late 1976. At this point he immersed himself in the punk rock scene with the band Swank (the remnants of the band Swankers, that became The Sex Pistols) as lead vocalist, until returning to theatre in 1978. Later he helped develop the musical production Up on the Roof, in which he starred in 1987 at London’s Donmar and Apollo theaters. He made his screen debut in 1979 as Rory Storm in Birth of The Beatles, and appeared in numerous British films and television programmes, including playing PC Dave Litten in the first series of The Bill, but achieved fame only with the role of Ben in the sitcom 2point4 Children (1991–1999). He played a starring film role as Arthur Hoyle alongside Samantha Janus and Neil Morrissey in the rugby league comedy, Up ‘n’ Under. He appeared in many TV adverts, notably the 1996 National Westminster Bank advertising campaign. In addition to his screen appearances he made numerous stage appearances including The Rocky Horror Picture Show and What the Butler Saw. He also played ‘Pope Liberty III’ in the Australian production of the controversial musical Bad Boy Johnny and the Prophets of Doom. Olsen appeared as Steve in The Comic Strip Presents…’ send up of The Fly, called The Yob. He also starred with Brian Bovell in the 1986 TV series Prospects about two lads in the Docklands trying to get ahead but usually failing.

Olsen played a small but pivotal role in the 1981 BBC TV series The Day of the Triffids as “Red-Haired Man”, a street gang leader, in episode 4 and again as an armed paramilitary officer named “Torrence” in the finale. He played PC Dave Litten in 12 episodes of The Bill between 1984 and 1986 (as well as the 1983 pilot episode, Woodentop). In 1988, he appeared in the EastEnders spin-off Civvy Street as Albert, the patriarch of the Beale family. His most prominent role was as Ben Porter in 2point4 Children (1991–1999). He also had leading roles in another two short-lived BBC sitcoms Health and Efficiency (1993–1995) and Pilgrim’s Rest opposite Gwen Taylor for one series in 1997. From 1999–2000 he played Johno in the Daz Dogs adverts and sponsors. Olsen also starred in the 1986 TV series Prospects about two men in the Docklands trying their hand at anything to make a “few bob” alongside Brian Bovell who played Jez Littlewood in Gimme Gimme Gimme. Olsen also appeared as ‘Tony’ in the 1993 BBC special ‘Paul Calf’s Video Diary’. Olsen was critically acclaimed for a number of his roles in musicals like The Rocky Horror Show, Cut and Thrust, Gorky Brigade, Welcome Home, The Pope’s Wedding, Saved Dialogues, Metamorphosis, Serious Money, What the Butler Saw, Way of the World, and Bad Boy Johnny and the Prophets of Doom. He received particular praise for his portrayal of Moey in On the Ledge at the National Theatre in 1993. Two years later he appeared in April in Paris at the same theatre. His last stage role was as Evan in Yasmina Reza’s Art, in 2000.