Monohan was part of Collingwood"s inaugural VFL season and also played for the club while they were in the Victorian Football Association (VFA). Renowned for his high marking, he was used mainly as a follower and half back. He was a premiership player with Collingwood in 1903, after missing out on selection in the 1901 Grand Final despite playing every game that season and in the 1902 decider through injury.
Monohan captained the Victorian state team between 1903-1906.
Monohan was inducted into the Collingwood Football Club Hall of Fame in 2012. He was the 53rd Collingwood player to receive the honour.
Monohan became a VFL goal umpire in 1913. After umpiring 181 VFL senior matches and three Grand Finals, Monohan retired as an umpire in 1926.