Boivin started the 1951–52 season playing for the Pittsburgh Hornets of the American Hockey League (AHL), before being promoted to the Leafs. The franchise had lost hard-hitting blue-liner Bill Barilko in the summer of 1951 when he disappeared on a fishing trip, and the Toronto brass felt Boivin's physical style could help fill that void. Boivin made his NHL debut for the Leafs on March 8, 1952. He only played in one other game that season, but featured regularly for the Leafs the following year. Early in the 1954–55 season, Boivin was traded back to the Bruins in exchange for Joe Klukay.

Back in Boston, Boivin enjoyed his best years as a feared member of the Boston Bruins blue line and helped lead them to two Stanley Cup Finals in the late 1950s. Boivin starred with other Boston defensive stalwarts Allan Stanley, Fern Flaman, Doug Mohns and Bob Armstrong. He remained the anchor of a youthful Bruins defensive corps during the difficult reconstructive period of the early 1960s. Boivin became captain of the Boston Bruins in 1963. In February 1966, he was traded to the Detroit Red Wings in a five-player trade. He recounted: "I had been in Boston for a long time and they were struggling. (General Manager) Hap Emms asked me if I would go to Detroit. It was near the (trade) deadline. (Doug) Barkley lost his eye that year and Sid Abel wanted me to go there. I said, 'Sure, I'll go to Detroit.' It was quite a feeling to go in there because they had guys like (Gordie) Howe and (Alex) Delvecchio that I had hit hard through the years. But they welcomed me with open arms. I really enjoyed playing there." That spring, Boivin helped the Red Wings to the Stanley Cup Finals, where they lost to the Montreal Canadiens.

After one more season with Detroit, Boivin's career was winding down but age 35, he was given a chance to extend his career when the league doubled in size by adding six new franchises for the 1967–68 campaign. Boivin found a new home when he was selected by the Pittsburgh Penguins in 1967 NHL Expansion Draft. He recalled how "after sixteen years in the six-team league, expansion added on to my career. At that time, there were a lot of players playing in the American Hockey League that were great players who just didn't get a chance." He spent a season and a half with the Penguins before moving on for a final time when he was traded to the Minnesota North Stars, another expansion club. After the North Stars were defeated in the quarterfinals of the 1970 playoffs, Boivin retired. He declined the opportunity to extend his career with the newly established Buffalo Sabres.

It was with the Bruins that Leo's reputation as a hard-hitting, defenseman grew. Tim Horton regarded him to be the toughest blue liner to beat in a one-on-one situation. He was nicknamed "Fireplug" for his compact stature and devastating body checks.

NHL Coaching record

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