St. Croix was drafted by the Philadelphia Flyers in the 4th round of the 1975 NHL Amateur Draft after a junior hockey career with the Oshawa Generals of the Ontario Hockey Association. He was also drafted by the World Hockey AssociationHouston Aeros in the 13th round of the 1975 WHA Amateur Draft. He had been a Second Team OHA All Star in 1973. St. Croix made his NHL debut for the Flyers during the 1977-78 season on February 16, 1978 at the Spectrum against the Minnesota North Stars. He played 7 games for the Flyers that season, while playing most of the season with the Maine Mariners of the American Hockey League. He played 2 games for the Flyers in 1978-79 and one game for the record-setting Flyer team of 1979-80. In 1979-80 he also won the Hap Holmes Memorial Award, awarded to goaltenders playing at least 25 games for the AHL team with the lowest goals against average, sharing the award with teammate Robbie Moore. He was also named a First Team AHL All-Star that season. 1980-81 was St. Croix's first full season in the NHL, in which he split time as the Flyers' goaltender with Pete Peeters and Phil Myre. He spent 2 1/2 years with the Flyers before being traded to the Toronto Maple Leafs during the 1982-83 season in exchange for goaltender Michel Larocque. St. Croix's final NHL season was 1984-85, which he split between the Maple Leafs and the AHL St. Catharines Saints. He spent one more season as a professional hockey player, 1985-86 with the International Hockey LeagueFort Wayne Komets. With the Komets that season, he shared the IHL James Norris Memorial Trophy with teammate Pokey Reddick, awarded to the goaltender having the fewest goals against during the season. He was also named an IHL Second Team All Star that season.
Coaching career
After retiring as a player, St. Croix became a coach. He served as an assistant coach for the Winnipeg Jets for the 1987–88 and 1988-89 seasons. He was later named goaltending coach for the Dallas Stars where he won the Stanley Cup with the team in 1999. St. Croix was goaltending coach for the Manitoba Moose from 2008 to 2011. Prior to the 2011-12 season, the team was relocated to Newfoundland and renamed the "St. John's IceCaps" where St. Croix continued to serve as an assistant coach/goaltending coach. In September 2012, with the resignation of Francois Allaire, St. Croix was hired as the Toronto Maple Leafs goaltending coach. St. Croix was fired in an organizational housecleaning on April 12, 2015.