A star of karting growing up, which included a national championship, Mostert dominated the 2010 Australian Formula Ford Championship, winning 10 of the last 12 races before claiming the title that year.
In 2011, he stepped up to the Dunlop Series, claiming a podium and pole position in his debut year. 2012 saw him finish third in the series, with only a points-penalty stopping him from claiming the championship.
After initially starting the 2013 season in the development series, he got the call-up to take over the #12 Dick Johnson Racing entry in the main series, where he saw immediate success, taking his first Supercars race win at Queensland Raceway.