McCartin originally played in the TAC Cup for the Geelong Falcons, but has also played three games in the AFL Under 18 Championships for Vic Country. He missed the whole of the 2016 season due to viral arthritis.[4] At the AFL Draft Combine, he recorded 88 centimetres (35 in) in the running vertical leap and level 21.5 in the yo-yo test.[5] McCartin was drafted with pick 33 in the 2017 draft while completing Year 11 at St Joseph's College, Geelong, but decided not to complete Year 12. He was the youngest player on an AFL list in round 1, 2018 at 18 years and 82 days.[6] Sydney coach John Longmire praised McCartin for his work rate during his first preseason.[2]
Tom is the brother of former St Kilda and Sydney player Paddy McCartin.[4] The brothers played against each other for the first time in round 12, 2018 at Docklands Stadium. Sydney won by 71 points and both brothers kicked a goal.[7] In September 2018, McCartin extended his contract by two years, tying him to Sydney until 2021. He played 15 games in his debut season as a key-position forward after Sam Reid was kept out with injuries. Football manager Tom Harley commented, "For an 18-year-old to hold down a key-position post for more than half the season in his debut year is an unbelievable achievement."[8]
^The 2020 season was played with 17 home-and-away matches per team (down from 22) and 16-minute quarters with time on (down from 20-minute quarters with time on) due to the impact of the COVID-19 pandemic.
