A product of the Narrabeen Sharks, Watmough first played in the NRL in 2002 for the Northern Eagles club. In 2005, Watmough was first selected for the New South Wales State of Origin team. Watmough was awarded the 2007 Dally M "Second Rower of the Year" award. He played in the 2007 NRL grand final defeat by the Melbourne Storm. He played in the 2008 NRL Grand Final victory over the Melbourne Storm and was then named in the Australian squad for the 2008 Rugby League World Cup.