He won the Australian Boys’ Amateur in 2016 and was twice winner of the New Zealand under-19 championship, in 2015 and 2016. Hillier was also twice winner of the Bledisloe Cup, in 2016 and 2018, as the leading amateur in the New Zealand Open, and was a co-medalist, with Cole Hammer, at the 2018 U.S. Amateur.
As a 17-year-old amateur, Hillier won the 2015 John Jones Steel Harewood Open on the Charles Tour. He beat Jim Cusdin by a stroke after finishing on 277, 11-under-par, with three birdies in his last five holes. He qualified for the 2019 U.S. Open through final qualifying at Walton Heath in England.
Hillier was part of the three-man New Zealand team in the 2018 Eisenhower Trophy in Ireland. The team led by three strokes after three rounds but faded on the final day and finished in fourth place behind Denmark, the United States and Spain. Hillier was tied for third place in the final individual standings with a score of 273, 17 under par.
Hillier turned professional in September 2019. In January 2020 he qualified for the PGA Tour of Australasia but after a few starts in early 2020, his year was affected by the COVID-19 pandemic and his appearances were largely limited to the Charles Tour. He had some success on that tour, winning two of the six events, the Brian Green Property Group NZ Super 6 Manawatu in March and the DVS Pegasus Open in October, and ending the year as the winner of the Order of Merit.
In 2021, Hillier qualified for the Open Championship after winning the final qualifying section at Notts Golf Club after rounds of 64 and 69, although he missed the cut in the championship itself.
Hiller has played on the Challenge Tour throughout 2021 & 2022, winning the Challenge Costa Brava in 2021 and the Swiss Challenge in 2022. He qualified to play the 2023 season on the DP World Tour after finishing seventh in the 2022 Challenge Tour’s overall season rankings.