Although born in Basingstoke, Mark played for Ireland through the parentage rule after being recommended to Jack Charlton. His first cap came on 27 April 1988 in a 2-0 friendly win over Yugoslavia at Lansdowne Road. Interestingly he won his first cap a month before he made his full debut at club level for Portsmouth at Manchester United in May 1988.
Alan Ball described the 18 year old Kelly as "the next George Best" but despite some exciting performances a series of injuries prevented him from becoming fully established in the first team. He never recovered from a serious knee injury sustained in 1991 and was forced to retire a year later, after a brief trial at Tottenham Hotspur FC in a bid to resurrect his career. Later he played part time for Sligo Rovers and Farnborough Town F.C..
He now works as head of youth development at his former club, Portsmouth.