Born in Southport, Queensland, Shelley made his first international appearance at the 2002 World Junior Championships in Athletics. He had set a career best of 9:08.28 for the 3000 metres steeplechase earlier that year but did not manage to progress beyond the heats at the competition. He soon changed his focus to the 1500 metres instead and set a personal best of 3:39.90 before going on to take third at the 2003 Australian Athletics Championships. He also ran in the short race at the 2004 IAAF World Cross Country Championships.
He emerged in the senior ranks in 2007, finishing in 65th place at the 2007 IAAF World Cross Country Championships and running a 3000 metres best time of 7:59.59. He began moving into longer distance events in 2008 and, after placing 50th over 12 kilometres (7.5 mi) at the 2008 IAAF World Cross Country Championships, he won the half marathon event at the Gold Coast Marathon in July, recording a time of 1:02:41. He earned selection for the 2008 IAAF World Half Marathon Championships and he was the region's best performer with a sixteenth-place finish. At the end of the year he competed in the 10,000 metres at the Zatopek Classic in Melbourne and finished in third place with a time of 28:08.96.
He broke new ground with personal bests of 13:38.30 for the 5000 metres and 27:59.77 for the 10,000 metres at the start of 2009. His third appearance at the World Cross Country Championships resulted in his best performance to date, coming in 36th place in the senior race. He returned to the Gold Coast half marathon to defend his title but he was pipped at the line by Tanzanian Dickson Marwa, leaving Shelley with a personal best of 1:02:10 and the runner-up spot just one second behind. In September he won the City2Surf in Sydney with a time of 41:02. However, the 2009 season also brought along some setbacks for Shelley's career. He suffered a stress fracture in his leg and he also lost his funding from the Australian Institute of Sport.
Making his debut in the event, Shelley entered the 2010 Rotterdam Marathon in April and finished in twelfth place with a time of 2:13:05. He represented Australia at the 2010 Commonwealth Games and was chosen for the marathon. Running in only his second outing over the distance, the relatively slow pace worked in his favour as he overhauled Amos Tirop Matui in the final stages to take the silver medal behind John Kelai.
He was invited to run at the 2011 London Marathon and he reached the top ten, setting a personal best of 2:11:38 hours. His second marathon race of the year came at the Amsterdam Marathon in October. Although he finished eleventh overall, he set a personal best time of 2:11:23 hours.
Running in bib number 1059, Shelley placed 16th overall in a time of 2:14:10 at the 2012 London Olympics. The race was won by Ugandan born runner, Stephen Kiprotich in a time of 2:08:01.
On October 13, 2013, Shelley placed 12th in the Chicago marathon with a time of 2:13:09 which was just outside his personal best of 2:11:23. The race was won by Kenyan Dennis Kimetto in a time of 2:03:45. In the process, Shelley became the 4th Australian marathon runner to clock an A qualifying time for the 2014 Glasgow Commonwealth games.
On 27 July 2014, Shelley won the Gold Medal in the Glasgow Commonwealth Games in a personal best time of 2:11:15.
On May 15, 2016, he won Sydney Morning Herald Half Marathon race in Sydney with a time of 01:03:59