Liam Millar joined the Liverpool's Academy in 2016 and can play as a centre forward or left winger. He became the club's first Canadian international when he made his debut for his country in March 2018. After spending most of the 2019 calendar year on loan at Kilmarnock, his Reds first team debut came in the FA Cup 4th round replay against Shrewsbury in February 2020. On 5 January 2021 Millar joined Charlton in League One for the remainder of the season on loan. In the summer he was transferred to FC Basel and established as the Swiss clubs first choice on the left of midfield the following season.

In 2022/23, Millar was part of a Basel squad that reached the semi finals of the Europa Conference League then was loaned to Preston North End for the 2023/24 season. He had an impressive season there, scoring five league goals from 35 appearances. He then featured in all six of Canada's games in the Copa America, where they reached the semi-final. Millar joined Hull City in August 2024, but sustained a cruciate ligament injury at the end of October which ruled him out for the remainder of the season.