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. Monahan was traded to the Pittsburgh Penguins in 1977, and would be dealt again during the 1977–78 season to the Los Angeles Kings. After a season in the minors, he was claimed by the Quebec Nordiques in the 1979 NHL Expansion Draft, and was traded soon after to the St. Louis Blues, where he played until he retired following the 1980–81 NHL season