Mulryne began his career as a youth team player at Manchester United in 1994. He was in the FA Youth Cup winning team of 1995, and became a Northern Ireland international even before he made his first team debut. His full international debut came in February 1997 against Belgium but Mulryne had to wait until the following October to make his debut for United against Ipswich in the League Cup (a 2–0 defeat). Despite being capable as a striker, midfielder or right winger, these positions were virtually monopolised by accomplished players such as David Beckham, Nicky Butt, Paul Scholes, Andy Cole and Ole Gunnar Solskjaer, and so Mulryne only managed a handful of first-team games in five seasons at Old Trafford. His only league appearance for United came on the last day of the 1997–98 season, in which he played the full 90 minutes against Barnsley.