How did Billy Joel get his name?

Billy Joel was born William Martin Joel.

During the spring of 1972, the Philadelphia radio station WMMR-FM began playing a concert recording of "Captain Jack", which became an underground hit on the East Coast. Herb Gordon, a Columbia Records executive, heard Joel's music and introduced him to the company. Joel signed a recording contract with Columbia in 1972 and moved to Los Angeles. For six months he worked at The Executive Room piano bar on Wilshire Boulevard as "Bill Martin", composing his signature hit "Piano Man" about the bar's patrons. He then began using the stage name Billy Joel later on.