Harkin began his career at a very young age with Don Boscos. Throughout his career with Don Bosco"s, Harkin made several appearances for the Derry and District League Select in the Foyle Cup and also County Londonderry in the Milk Cup. Having turned down opportunities to sign for a number of English clubs, such as Blackburn Rovers, in order to finish his GCSE exams at Saint Columb"s College, he eventually signed for Charlton Athletic in the summer of 2006.
Harkin looked set to have a bright future in the game and his ability was praised by coaches.
However, the young midfielder failed to play a senior game for the English team and instead languished in the reserves, before being loaned out to non-league outfit, Heybridge Swifts. At the end of 2008, Harkin was released by Charlton and promptly snapped up by his hometown club, Derry City.
He made 12 appearances for Harps in the early part of 2009 before returning to the Brandywell where he turned out 3 times for the Candystripes before the season ended. Harkin signed for Coleraine in the Irish Premier League on 5 January 2012.
Having represented Northern Ireland at Under-16 level, the Derry-born Harkin was later called up to the Republic of Ireland Under-19 squad, where he was capped 4 times between 2006 and 2008.