 Born in Montreal, Quebec, Monahan's father-in-law is the late Hall of Famer Bernie "Boom Boom" Geoffrion, and is the father of former Major League Baseball player Shane Monahan.Selected by the California Golden Seals in the 1971 NHL Entry Draft, Monahan played only one game with the Golden Seals before he was traded to the New York Rangers.During an Intra-League Draft in 1975, he was claimed by the Washington Capitals, where he played for two seasons.