Behan makes his Rovers debut on February 8, 1931 in a 5–1 win over Bray Unknowns F.C. at Glenmalure Park. In his first season, he wins the FAI Cup. He signs with Manchester United in September 1933, along with fellow Irishman David Byrne, becoming the first players from the south of Ireland to play for the club in over a decade.
Behan makes his United debut in a Football League Second Division home game against Bury F.C. on March 3, 1934. The following July, he briefly returns to Shelbourne before again returning to the Rovers. Over the next two seasons, he wins another FAI Cup and a League of Ireland Shield. His last game for the Rovers is on August 23, 1936 in a Shield win over Drumcondra F.C..
After his retirement as a player, Behan becomes a respected referee and is in charge of the 1943 FAI Cup Final. He then manages Drumcondra F.C. in the 1950s where he wins the FAI Cup again.
Behan subsequently becomes United’s chief scout in the Republic of Ireland and is credited with discovering, among others, Johnny Carey, Billy Whelan, Tony Dunne, Don Givens, Kevin Moran and Paul McGrath. He also serves as vice chairman of the Dalkey-based Leinster Senior League team, Dalkey United, and it is through this association that he discovers McGrath.
Behan’s father, William Sr., is one of the founder members of Shamrock Rovers. His brothers John and Paddy also play for the Rovers. His son William junior keeps goal for the Rovers side also for a time. His second cousin is Bob Fullam.
Behan’s grandson, Philip Behan, is the former Head of International Football at the Football Association of Ireland and is now a UEFA and FIFA agent organising friendly international matches and tournaments around the world.
Billy Behan dies on November 12, 1991 at the age of 80.