McKillen first played hurling with the McQuillan Ballycastle club. He was still eligible for the minor grade when he won his first Antrim SHC medal after a defeat of Loughgiel Shamrocks in 1983. McKillen won a second title a year later when McQuillan's retained the title, before claiming a third and final winners' medal in 1986. Each of these three victories were subsequently converted into Ulster Club SHC title successes.[2]
At inter-county level, McQuillan first appeared for Antrim at underage levels, however, his minor and under-21 tenures ended without any title success. McKillen made his senior team debut in 1984 and went on to win seven Ulster SHC titles in a ten-year period between 1989 and his retirement in 1998.[3] He also lined out at midfield when Tipperary beat Antrim by 4-24 to 3-09 tin the 1989 All-Ireland final.[4]
McKillen first became involved in coaching as a result of his day job as a caretaker at Cross & Passion College. He has served as a coach and selector with various college teams and was part of four Mageean Cup-winning management teams. McKillen later became involved in inter-county management as a selector with the Antrim senior hurling team that won the 2006 Christy Ring Cup after a defeat of Carlow in the final.[6]