He made his NHL debut with the Canadiens in the 1967–68 season, but spent most of the year with Montreal's Central Hockey League affiliate, the Houston Apollos.
After spending almost the entire 1968–69 season in the American Hockey League with the Cleveland Barons, Monahan was traded to the Detroit Red Wings in June 1969 in the deal where the Canadiens acquired Pete Mahovlich.
Monahan saw little ice time and struggled offensively and before the end of the season was traded to the Los Angeles Kings, where the story was the same.
Before the next season, Monahan was dealt to the Toronto Maple Leafs in the trade where the Kings acquired their future captain and coach, Bob Pulford.
After his offensive numbers dropped off in his final year in Vancouver, Monahan rejoined the Leafs for the 1978–79 season, scoring just four goals in 62 games.