He was a prominent squad member of the 2010 premiership-winning St. George Illawarra team, and came from the bench to play in the 2010 NRL Grand Final. In May 2011, he was selected on the bench for New South Wales for game 1 of the 2011 State of Origin series. He was selected for the second match but was then dropped for the third. On 2 March 2015, Merrin signed a five-year contract with the Penrith Panthers, starting in 2016.