He was born Lionel Begleiter in London, of Jewish extraction, and grew up in Stepney. He first made his name (which he changed to Bart after St Bartholomew's Hospital) with the 1959 musical, Lock up your Daughters, based on an 18th century play by Henry Fielding. His next hit, Fings Ain't Wot They Used T'Be, was notable for encouraging the use of authentic Cockney accents on the London stage, and Bart became a "trendy" figure. He surpassed himself with Oliver! (1960), based on Dickens' Oliver Twist.
Bart's later musicals, including Blitz! (1962) and Twang! (1965), never lived up to his early promise.