Cumbrian defender Harkness was snapped up by Kenny Dalglish for £75,000 from hometown club Carlisle United in 1989. Dalglish had been impressed with the youngster's performance against the Reds in an FA Youth Cup tie where he had caught the eye playing as a striker.
He began his career in the reserves in midfield but was quickly converted to full back. He had to wait two years before being handed a first team debut in August 1991 by Graeme Souness who had recently replaced Dalglish.
Harkness was way down the pecking order for the left back spot, having to go up against the likes of David Burrows, Steve Staunton, Julian Dicks and Stig Inge Bjornebye. Frustration got the better of him and he asked Roy Evans for a transfer in the 1994-95 season. A loan spell at Southend followed, but he stuck around and eventually he got his break at the end of that season when an injury crisis opened the door for him.
He did well enough to earn a new contract and was a regular in the side the following season, playing mostly as a wing back in Evans' 3-5-2 formation. He was beginning to establish himself as a first team player when a rash challenge from Coventry winger John Salako shattered Harkness's leg and sidelined him for almost a year.
He battled back and forced his way back into Evans' plans, this time slotting into the side on the left of the back three.
The arrival of Gerard Houllier signalled the end for 'Harky', the French boss wasted little time in showing him the door when he became sole manager after an ill fated joint manager experiment with Roy Evans. The player who had joined the club for £75k almost a decade earlier left for ten times that amount to join former boss Souness at Benfica.
He didn't last long in Portugal however, Souness was sacked a month after Harkness arrived and the defender fell out with the club president as a result, returning to England to join Blackburn Rovers, where he would soon be re-united with Souness who took over at Ewood Park seven months later.
Six months later Harkness was on the move again, spending two years at Sheffield Wednesday before linking up with former team-mate Mark Wright at Chester City. Injuries were beginning to catch up with him however and in July 2002 he hung up his boots at the age of 31.