As a graduate of the "Tobans" Athletic Club, Bill Mosienko tasted victory early in his career as a member of the "Paperweight" city playground champions. From the Tobans, Mosienko moved to the Sherburn Athletic Club where he played flyweight, midget, and juvenile hockey. In 1939, Mosienko then joined the Winnipeg Monarchs Junior Team.
Mosienko signed a contract with the Chicago Black Hawks for the 1942-43 season and entered their farm system where he spent half the season with Providence and Kansas City before finishing the season with the parent club. The Second World War interrupted the careers of many National Hockey League players and after only three games of the 1942-43 season, Mosienko enlisted in the armed forces and spent the rest of the season in Quebec where he worked in the shipyards and played for the Morton Aces hockey team. In 1943-44, Mosienko returned to the Chicago Black Hawks for whom he toiled faithfully for the next fourteen NHL seasons. Mosienko was named to the all-star teams in 1944 and 1945 and was also awarded the Lady Byng Trophy for sportsmanship and hockey excellence in 1945. The National Hockey League record books include one of his greatest accomplishments when he scored three goals within twenty-one seconds in New York against the Rangers on March 23, 1952.
Following his final season with Chicago in 1954-55, Mosienko joined the Winnipeg Warriors of the Western Hockey League where he played for four years and ended his career as coach of the club in 1960. Mosienko played 711 games in the National Hockey League and scored 258 goals with 282 assists. He was inducted into
the Hockey Hall of Fame in 1965.