Born in Scunthorpe, Lincolnshire, Harsley started his career at the Grimsby Town youth system as a trainee and signed a professional contract on 16 July 1996.

[3] He broke into Scunthorpe's first team on 20 September 1997, aged 18, in a 1–0 defeat at Barnet, replacing Alejandro Calvo García as a late substitute.

He maintained his first-team spot in 2000–01, but was released by the club at the end of the season,[5] subsequently joining Halifax Town on a free transfer on 1 July 2001.

[3] He spent one season at Halifax, which proved to be highly successful on an individual level, with Harsley ending up as their top scorer with twelve goals from fifty games; however, the team performed extremely poorly, and they were relegated out of the English Football League, a full nine points below Bristol Rovers.

[8] He was highly productive throughout the 2002–03 season, making 51 appearances in all competitions and picking up the club's Player of the Year award.

[9] A successful season followed in 2004–05, when his club made the play-offs, finishing ahead of eighth place Darlington on goal difference.

[14] At the end of the season, he was out of contract,[15] and decided to move on after turning down a new deal at Vale Park and instead signed for Chesterfield.

[17] After starting the 2009–10 campaign just as poorly, he signed for Darlington on loan for three months on 26 November 2009, along with teammate Danny Hall.

He joined Conference Premier side York City on loan until the end of the season in March 2010,[19] teaming up with his former Vale manager Martin Foyle.

[28] On 6 February 2018, Harsley was named as caretaker manager at Barnsley following Paul Heckingbottom's move to Leeds United.

[29] He managed the South Yorkshire derby between the "Tykes" and Sheffield Wednesday four days later, leading the team to a 1–1 draw at Oakwell.

[34] After John Eustace took over from Bowyer, Harsley initially remained in post before leaving in August 2022 for Huddersfield Town, where he was appointed one of two assistants to head coach Danny Schofield.

[38] On 27 November 2022, it was announced that Harsley had left Huddersfield after Fotheringham had brought in Kenny Miller to be his new co-assistant coach.