He made his Derry Senior debut in 2004, but did not make his Championship debut until Round 2 of the Qualifiers against Kildare in July 2006.[4] Mullan missed most of the 2007 season, due to being in the United States, but has returned for the 2008 campaign. He helped Derry reach the 2008 Dr McKenna Cup final, where they were defeated by Down.[5] He was part of the Derry team that won the 2008 National League where Derry beat Kerry in the final.[6]