When Sweeney was 11 years old, he moved to London with his parents, along with two brothers and a sister. He became interested in theatre and improvisation while attending Clapham College, an all-boys Roman Catholic school. Sweeney spent the 1970s in theatre productions, creating and touring shows with Steve Steen, who was a friend from school. The pairing's first television work as a duo came on the ITV children's show CBTV, followed by the Channel 4 comedy Little Armadillos and resident support comic slots on Rory Bremner's first sketch show for the BBC. In 1987, Sweeney played Samuel Taylor Coleridge in an episode of Blackadder the Third, with Steve Steen alongside him as Lord Byron. He also starred as "The Head" in the first season of Art Attack in 1990. He then appeared in commercials and radio comedies before acquiring a semi-regular slot as a contestant on the Channel 4 improvisation show Whose Line Is It Anyway? from 1991 to 1993. Steve Steen also occasionally appeared on Whose Line alongside Sweeney.
Since 1985, Sweeney has had multiple sclerosis,[1] something which has required use of a walking stick and, from 2005, a wheelchair.[citation needed] Sweeney wrote about his experiences of multiple sclerosis in My MS and Me,[1] a play he performed for BBC Radio 4 after a successful Edinburgh Festival run. In 2008, the disease meant he gave up appearing on stage, but he continues to be listed as a member of the Comedy Store Players.[citation needed] In 2007, he made two series of BBC Radio 4's improvised sketch show The Lawrence Sweeney Mix with Josie Lawrence.[2][3][4]