McMullin ran the second-fastest 20m sprint at the 2022 draft combine, at 2.88 seconds, and Greater Western Sydney took him at pick 34 that year. He debuted against Port Adelaide in round 22 of 2023 and has since been used through the midfield and ahead of the ball. His father Ian played for Essendon and Collingwood before spending two decades on the Magpies' board. He has played 36 AFL games and kicked 8 goals.