William T. Reay began his career with the East Kildonan Bisons of the Manitoba Midget League. After 2 seasons with the Bisons and two more with the Elmwood Maple Leafs Juvenile Team, he graduated to the St. Boniface Seals of the MJHL. He helped the Seals win the Memorial Cup in 1938.

He began his pro career with the Omaha Knights of the AHA, winning Rookie of the Year Honours in 1939-40. He returned to the amateur ranks and was player/coach of the Quebec Aces as they won back to back Allan Cup titles. Reay won the Quebec Senior League Scoring Title, was named all-star centre and won the Lady Byng Trophy. After a pair of short stints with the Detroit Red Wings, Reay joined the Montreal Canadiens and helped them to the 1945-46 Stanley Cup. He played 7 more seasons in Montreal, culminating his NHL playing career with another Stanley Cup in 1952-53. During 10 NHL seasons, Reay played 479 games (105 goals, 162 assists, 267 points) and in 63 playoff games (13 goals, 16 assists, 29 points).

In 1953, Reay turned to coaching. He spent 2 years with the Victoria Cougars (Western League) and another season in Rochester of the AHL, where he was coach of the year, before being hired by the Toronto Maple Leafs in 1957. After 2 seasons with the Leafs, it was back to the AHL, where led the 1962-63 Buffalo Bisons to the Calder Cup. He was hired to coach the Chicago Blackhawks in 1963 (14 seasons), the Hawks finished first 6 times and he coached in 3 NHL All-Star Games. His 542 victories earned Reay a milestone award from the NHL.

A member of the Manitoba Hockey Hall of Fame and the Chicago Hall of Fame, Billy Reay is one of the few players in hockey history to have played on Memorial, Allan and Stanley Cup Championship Teams.