Horsley was born and raised in Kalgoorlie, in the Goldfields region of Western Australia. He excelled in a number of sports at junior level, playing A-grade field hockey and cricket, representing Western Australia at hockey at under-15 and under-16 level. Horsley made his debut for the Kalgoorlie City Football Club in 2003, at the age of 16, and won the Mitchell Medal as the best player in the Goldfields Football League in 2006. Despite wishing to remain in Kalgoorlie, he was recruited to the Subiaco Football Club in the West Australian Football League in 2007. Horsley made his debut against in round 16 of the 2007 season, kicking two goals and recording 21 disposals, and played a total of 10 reserves games and seven senior games during the season. The following season, he played 21 games mainly as an inside midfielder, winning the Football Budget's award for the Best First Year Player, and tying with Adam Cockie for Subiaco's Rookie of the Year award. Horsley made his state debut for Western Australia in 2008 against Queensland in Townsville, and also played in the interstate game in 2010. In 2011, he played 23 games, including the grand final loss to, and won the Tom Outridge Medal as Subiaco's best and fairest player, as well as finishing runner-up in the Sandover Medal to Luke Blackwell as the best player in the league. Speculated as a likely selection in either the 2011 National Draft or the 2012 Rookie Draft, Horsley was eventually taken by with the second pick in the Rookie Draft. He had previously been invited to train with, prior to the start of the 2008 season, but was not drafted. Horsley spent the first few rounds of the season with Gold Coast's reserves team in the North East Australian Football League, but was elevated from the rookie list to the senior list after Jarrod Harbrow received a long-term injury. He made his debut the following week against at Docklands Stadium, replacing the injured Gary Ablett, and recorded 19 disposals and eight tackles. Overall, Horsley played 13 games in his first year, and was considered one of the club's better players throughout the latter half of the season, with The Gold Coast Bulletin noting that he provided "that dash of maturity so needed in a young club". At the club's Club Champion Awards held at the end of the season, Horsley received both the "Most Professional" award and the "Iron Man" award. He was upgraded to Gold Coast's senior list for the 2013 season in October 2012, during the 2012–13 off-season. Horsley played only senior game during the 2013 season, after rupturing his anterior cruciate ligament midway through the season. He was delisted by the Suns at the end of the season, and returned to Perth at the end of the season. Horsley returned to the WAFL with the Subiaco Football Club where he was announced captain in the pre-season. Horsley recovered from his anterior cruciate ligament injury for Round Four against Peel; kicking the first goal in a 17-point win over the Thunder. He amassed 499 disposals from 19 games on the season as well as leading Subiaco to their 12th WAFL premiership over the East Perth Royals.
