Randy St. Claire's journey in Major League Baseball began when he was drafted by the Montreal Expos in the 1980 amateur draft. As a right-handed pitcher, he made his MLB debut with the Expos on September 11, 1984. St. Claire's early years in the league were marked by his role as a relief pitcher, where he demonstrated his ability to handle pressure situations on the mound.

Throughout his career, St. Claire played for several teams, including the Cincinnati Reds, Minnesota Twins, Atlanta Braves, and Toronto Blue Jays. His tenure with the Expos was the longest, spanning parts of five seasons. During this time, he developed a reputation for his work ethic and adaptability, often being called upon in various relief roles. His time with the Expos saw him posting a career-best ERA of 3.12 in the 1985 season, showcasing his potential as a reliable bullpen option.

St. Claire's stint with the Cincinnati Reds in 1988 was brief, as he appeared in only a handful of games before moving on to the Minnesota Twins. With the Twins, he continued to serve primarily as a reliever, contributing to the team's bullpen depth. His ability to pitch multiple innings when needed made him a valuable asset to the teams he played for.

In 1991, St. Claire joined the Atlanta Braves, where he was part of a team that would go on to win the National League pennant. Although his role was limited, being part of a pennant-winning team was a highlight of his career. His final MLB appearance came with the Toronto Blue Jays in 1994, marking the end of a decade-long career in the major leagues.

After retiring as a player, St. Claire transitioned into coaching, where he continued to share his knowledge and experience with younger players. His post-playing career included roles as a pitching coach for various teams, including a notable tenure with the Washington Nationals.