Although an inaugural member of the club, Harley struggled to force his way into the senior team and spent the majority of his early years playing for Norwood in the South Australian National Football League (SANFL).

Harley quickly established himself within the Geelong defence and received a range of accolades and club honours, including multiple Best Clubman awards.

Since his retirement as a player, Harley has undertaken various roles in the sporting industry: he held the position of "General Manager - Football" with the Sydney Swans, and became their CEO in 2019.

[3] Harley's persistent attendance at training was rewarded when he was called up to the under–18s squad as a late injury replacement a week prior to the National AFL Championships.

[6] Harley continued to ply his trade in the SANFL reserves during the 1998 season, before being rewarded mid-season with a senior team debut for Port Adelaide in round fourteen against Geelong.

Harley made his debut for Geelong in round fourteen of the 1999 AFL season against Adelaide, and kept his position in the senior team for the final nine games of the home-and-away campaign.

After playing the majority of his career at Geelong as a centre half back, Harley struggled to find his niche in the defence following the emergence of the younger Matthew Egan in the key position.

He soon adjusted himself as a floating defender capable of playing on small forwards, and retained his position in the side after achieving career-best averages of 14 disposals and 5 marks a game.

[10] Harley helped the team compile an 18–4 win–loss record at the conclusion of the home and away season, earning them their eighth McClelland Trophy and securing first position on the ladder leading into the finals series.

[3] Harley's leadership qualities and influence on the club as captain was commended early on in the season after his handling of teammate Steve Johnson's off-field discretions.

He set career-highs of 14 marks (round three) and 14 handpasses (Preliminary Final) in the process,[8] and helped the club achieve the number one defensive record[11] during the season for the second consecutive year.

Following the failure of 2008, Harley suggested that the club had learned that it needed to "just be the best team in September"[14] and that once qualified for the finals, focus had shifted towards rest and recovery.

[12] In the weeks following his retirement, Harley was sounded out by the AFL's newest team, Greater Western Sydney Giants, to join the club in an off-field role.

On 4 December 2009, it was announced that Harley would join the club as a project consultant, serving on committee board and the football department in a part-time role.

[19] Prior to the 2010 AFL season, it was further announced that Harley would join the Channel 7 network football commentary team, replacing Nathan Buckley in the leading special comments role.

[20] Harley admitted that his previous experience as a regular panellist on the football program One Week at a Time[21] during his playing days had fuelled an interest in working within the media industry.

[23] Harley began his early career as a key position player at centre half back, where he used his physical size to great effect.

[10] Despite his personal admission to not being "the greatest player",[3] Harley's play-reading ability saw him recognised as one of the league's best at intercepting opposition passes inside his defensive 50m arc.

[16] Harley's legacy as captain of the club was pronounced by Bill McMaster as being one of the great leaders of Geelong alongside Reg Hickey and Fred Flanagan.

[26] Harley was one of 21 players from the Geelong Football Club who witnessed the 2002 Bali bombings first-hand during an off-season holiday to the popular tourist destination.

[28] As part of his ambassadorial role, Harley has appeared in numerous advertisements alongside fellow Geelong teammates and ambassadors James Kelly and David Wojcinski.