Les Costello was born in South Porcupine, Ontario on February 16th 1928. Costello's professional career began in 1947 with the American Hockey League's Pittsburgh Hornets and the left-winger scored 32 goals as a rookie. That spring he made his NHL debut when he joined the Toronto Maple Leafs for the playoffs.
With the Leafs Costello played five games and scored 2 goals and four points. The Maple Leafs went on to win the Stanley Cup that playoff. The following year Costello made his regular season debut dressing for 15 games with Toronto and posting 5 points. The rest of that season and all of the following campaign was spent back with the Hornets but Costello did have one final NHL game to play. After spending the entire year in the minors, the Leafs summoned Costello for a playoff appearance in 1950. He played one scoreless contest for the Maple Leafs that spring. It would prove to be Costello's last professional hockey game. That year he retired from hockey to pursue Seminary studies.
After becoming a catholic priest, Costello continued to play the game with "The Flying Fathers" an all-priests hockey team.