Greenhalgh, from Orpington, who played for the Welling Unitedyouth team, was featured in the Sky One show Football's Next Star and impressed judges in the trial game after laying on a perfect cross for Hicham Abdellah to head home. At the end of the opening episode, the judges presented the Inter Milan shirt to ten players who would travel to Italy to compete for their dream of a professional contract in later episodes. After 20,000 entries to the show and eight episodes, Greenhalgh was given the shirt and revealed as the winner of Football's Next Star 2010, receiving a professional six-month contract with Inter Milan.
After his six-month professional contract at Inter Milan came to an end, Greenhalgh was offered another year from the Italian giants which included a four-month loan spell to Calcio Como, where he scored eight goals from 12 games. During his time with Inter Milan, Greenhalgh was presented with a Champions League Winners medal. After his contract was up, he returned to the UK and spent time at Brighton & Hove Albion, as well as a spell at Birmingham City, where he was advised to play first team football in the Conference to gain experience in the English game. He signed for non-League club Welling United in December, before later joining Ebbsfleet United and Maidstone United. In June 2013 he transferred to Scottish club Inverness Caledonian Thistle. This was due to happen in February 2013, however international clearance stopped Greenhalgh from leaving England until the 2013–14 season. Having played only six first-team games for Inverness, Greenhalgh was loaned to Scottish League One club Stenhousemuir for the final six games of the season. In June 2014 it was reported that Inverness had opted not to offer Greenhalgh a further contract, with manager John Hughes stating that he could not guarantee Greenhalgh first-team football. He currently coaches at football training camps, run by Skills Academy Director Mike Delaney It was announced on 22 June 2015, that Greenhalgh had signed for National League South side Concord Rangers, returning to a club where he had made a big impact during the run-in of the 2012–13 season, winning both the Isthmian League Cup, and the Isthmian League Premier Division play-offs. On 26 May 2016, it was announced that Greenhalgh had signed for Maidstone United again. He was released by Maidstone a year later in May 2017. He was immediately signed by National League South side Hemel Hempstead Town, who had Greenhalgh on loan towards the back end of the 2016–17 season. On 24 May 2018, it was announced that Greenhalgh had signed for Dartford. On 8 November 2019, it was announced that Greenhalgh had signed for Tonbridge Angels on an initial month's loan. His loan was then extended until 4 January 2020. On 7 January 2020, it was announced that Greenhalgh had left Dartford by mutual consent. The following day, Greenhalgh signed permanently for Tonbridge Angels.
