Masterton began his career at Kilmarnock, but didn't play for the senior team.
Masterton signed for Clyde in the summer of 2005, and made his début as a substitute in a Scottish Challenge Cup tie against Brechin City. Masterton became a fringe player in his first few months at Clyde, but got a regular place in the team, filling in for long term injury victim Paul McHale. Masterton also won his midfield battle with Roy Keane during Clyde's famous win over Celtic in the Scottish Cup, in January 2006.
Masterton is somewhat of a dead-ball expert, and the majority of his goals come direct from free kicks. He was Clyde's second top scorer in season 2006–07, scoring 8 goals.
At the end of season 2007–08 during Clyde's Scottish First Division semi final play-off first leg match at Alloa Athletic, Masterton scored what proved to be a vital. In a 2–1 defeat, he scored the equaliser with a trademark free kick just after the hour mark . Clyde went on to win 6–5 on aggregate.
On 16 May 2008, Masterton agreed to join First Division rivals Greenock Morton.
'Masty' scored his first goal for Morton in a 3–0 Challenge Cup win over Jim McInally's East Stirlingshire before doubling his Morton tally with a 25-yard free-kick in a 4–3 cup shock victory over Scottish Premier League side Hibernian. He continued his cup goal scoring exploits with the opener against Peterhead in the Scottish Cup loss at Balmoor, before opening his league tally with the equaliser first half free-kick at Cappielow in the victory over Ross County.
He was released by Morton at the end of the 2009–10 season.
In June 2010, Masterton joined Conference National side Crawley on a one-year deal.
In the January 2011 transfer window, Masterton was loaned out to Eastbourne Borough.
After his release by Crawley, Masterton signed for Auchinleck Talbot on an amateur contract, to allow to sign for a senior side if one offered him terms.
After one match in the Juniors, Masterton was offered a contract until December with Alloa Athletic in the Third Division. He made his début in the Scottish Cup draw with Annan Athletic, and he scored the final goal of the match ten minutes after replacing another ex-Morton midfielder, Darren Young.
In August 2012, Masterton made his second début for Clyde as a trialist during the club's opening league game of the season at Montrose winning 3–2. Following two more appearances for the club as a trialist he signed a short-term deal with Clyde until the end of 2012. On 10 November 2012, Masterton left the club by mutual consent after he injured himself in a Scottish cup game with Nairn County, The Clyde website said he was suffering with groin problems.
Masterton signed for Junior side Hurlford United in early 2013 after an unsuccessful trial at Troon.