Flack began karting in 2011 at the age of five, entering the Bambino Queensland Cup and winning the competition in both 2011 and 2012. By 2014, his early talent had attracted the attention of CRG, who signed him as a factory driver, enabling him to compete internationally alongside his domestic Australian programme.
In 2016, Flack claimed victory at the Macau International Kart Grand Prix, a prestigious international karting event, and that same year began competing in World Series Karting. Switching to the Energy Course team in 2019, Flack won the IAME International Final in the Junior ROK category and also competed in the FIA European Karting Championship in the OK Junior class.
In 2020, Flack was selected as one of two drivers representing Asia, Pacific and Oceania at the Ferrari Driver Academy scouting world final, sharing that honour alongside compatriot James Wharton. He reached the final of the evaluation event, marking him as one of the most promising young talents from the region.
That same year, before making his single-seater debut, Flack spent time in New Zealand working as an engineer in the Toyota Racing Series, giving him an early understanding of the mechanical side of racing from a technical perspective.
Flack made his car racing debut in the 2021 F4 British Championship driving for Phinsys by Argenti. He had participated in official British F4 tests in 2020 as well as tests with MTEC Motorsport before his competitive entry. He also competed in select rounds of the ADAC Formula 4 and the Italian F4 Championship during the year, finishing eleventh in the British F4 season with three overall podiums.
For 2022, Flack joined R-ace GP for his first full season in the Italian F4 Championship. He also contested the Spa-Francorchamps round of ADAC Formula 4. Mid-season he returned to the British F4 Championship from Round 9, this time with Fortec Motorsports.
Flack made his GB3 Championship debut in 2022 with Douglas Motorsport, competing alongside Max Esterson and Tommy Smith. He made an immediate impression, claiming his first podium during Race 3 at the Snetterton round. He then took his first GB3 win during Race 3 at the second Silverstone Circuit round of the season, demonstrating competitive pace in the higher formula.
He subsequently continued his involvement with Douglas Motorsport in GB3, building on the experience gained across multiple championships in his debut car racing seasons.
At the end of the 2022 season, Flack joined Jay Howard Driver Development for the final two rounds of the Indy Pro 2000 Championship in the United States. Competing as a guest driver and therefore ineligible for championship points, he achieved a best finish of fourth and ended the partial campaign seventeenth in the standings, gaining valuable experience of American open-wheel racing.
Flack's progression from Queensland kart racing to international junior formulas follows a path enabled by his family's motorsport background and his early factory karting relationship with CRG. His dual-hemisphere career — combining European single-seater programmes with occasional forays into American racing — reflects the approach of drivers seeking broad exposure across the world's main junior single-seater ladders. His coaching relationship with Dan Cammish has been cited as an influence on his technical and racecraft development through the junior formula years.