He played 14 seasons in the National Hockey League (NHL) from 1973 to 1987, with the Buffalo Sabres, Boston Bruins, Vancouver Canucks, and New Jersey Devils.

In the early 1970s NCAA players rarely made it to the National Hockey League, but McNab was drafted by the Buffalo Sabres while playing forward for the Denver Pioneers in 1972.

McNab led the Cincinnati Swords of the American Hockey League (AHL) in scoring in 1973–74, despite playing in 49 of 76 games, and debuted with Buffalo that same season.

On December 23, 1979, during a game at Madison Square Garden in New York, McNab, teammate Mike Milbury, and several other Bruins climbed into the stands to confront fans.

His father Max McNab was a centre who won the Stanley Cup with the Detroit Red Wings in 1950, and later became a coach and general manager.

Peter's brother, David, spent 43 seasons as an NHL scout and executive before retiring from his position as the senior vice president of hockey operations for the Anaheim Ducks in May 2021.