Mostert grew up in Browns Plains near Brisbane and began kart racing at age seven in Ipswich and the Gold Coast. He transitioned to Formula Ford in 2008, finishing eleventh in his debut season and fourth in 2009 before winning the 2010 Australian Formula Ford Championship with a record number of race wins in a single season.
In 2010 Mostert also drove a V8 Supercar at Bathurst in the Fujitsu V8 Supercar Series, finishing sixth and fourth in the two races. Ford Performance Racing signed him for the final two rounds of the 2011 Development Series at Sandown and Homebush, where he took pole on debut and added three race podiums. He won the Mike Kable Young Gun Award at the end of 2011. In 2012 he won the Adelaide 500 round of the Development Series with two race victories and finished third in the championship. On 1 December 2012 he was awarded an honorary commission as a Lieutenant in the Royal Australian Navy.
Mostert made his Supercars debut at the Perth 360 at Barbagallo Raceway in 2013 for Dick Johnson Racing, replacing Jonny Reid. At only his fifth championship event, the Ipswich 360 at Queensland Raceway, he took his first race win. He finished 17th in the championship despite missing the opening rounds.
Joining Ford Performance Racing in 2014, Mostert won the Bathurst 1000 with co-driver Paul Morris after overtaking a fuel-starved Jamie Whincup on the final lap. The win was remarkable for its circumstances: Mostert and Morris had been excluded from qualifying and started at the back of the grid, the lowest ever starting position from which Bathurst had been won to that point. He finished seventh in the championship.
In 2015, with the team rebranded as Prodrive Racing Australia, Mostert was in championship contention when a horrific crash in Friday qualifying for the Bathurst 1000 at Forrest's Elbow left him with a broken leg and broken wrist, ruling him out for the remainder of the season. His wrecked car was subsequently preserved at the National Motor Racing Museum near the circuit.
Mostert returned with Rod Nash Racing (a satellite of Prodrive) for 2016 and 2017, winning at Phillip Island, Queensland Raceway, and the Gold Coast 600 in 2017 — the latter with co-driver Steve Owen, earning the first Enduro Cup for both the team and for Ford. When the outfit was renamed Tickford Racing in 2018 he continued there through 2019, winning at the Gold Coast 600 in 2018 and at Albert Park in 2019.
Mostert moved to Walkinshaw Andretti United for 2020, switching from Ford to Holden. He signed a long-term contract extension in June 2021 and won the Darwin Triple Crown shortly after. In November 2021, during the second Sydney SuperNight race, he drove from 26th on the grid in wet conditions to finish on the podium in third place. That year he also won the Bathurst 1000 with Lee Holdsworth and set a new Bathurst lap record of 2:03.3736 in the Top 10 Shootout.
He was crowned 2025 Supercars Champion on 30 November 2025.
Mostert's GT career began with entries in the GT3 class of the Bathurst 12 Hour from 2017. In 2018 he joined the factory-supported Schnitzer Motorsport BMW programme and took the Allan Simonsen Pole Position Trophy. He also won the inaugural Bathurst 6 Hour in 2016, sharing a BMW 335i E92 with Nathan Morcom in the Group 3E Series Production Cars category.
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