*Club domestic league appearances and goals, correct as of 23:32, 15 January 2025 (UTC) ‡ National team caps and goals, correct as of 02:24, 20 October 2019 (UTC)
Aitchison made his first team debut on 15 May 2016, becoming the youngest player ever to represent the club in a competitive match at 16 years and 71 days. He scored with his first touch in the match, providing Celtic's seventh goal in a 7–0 win over Motherwell, in turn making him the youngest ever player to score a competitive goal for the club.[7][8] His next appearance for the first team came in January 2017, coming on in the final minutes of a home league match against Hearts and winning a penalty which Scott Sinclair converted for Celtic's last goal in a 4–0 win.[9]
On 2 September 2019, Aitchison moved on a season-long loan deal to English League Two club Forest Green Rovers.[12] He scored on his debut as a substitute in a 1–0 win away to Cambridge United on 7 September 2019.[13]
Barnsley
On 5 October 2020, Aitchison joined Championship side Barnsley on a three-year contract after leaving Celtic.[14]
Forest Green Rovers (loan)
On 2 July 2021, Aitchison returned to former loan club Forest Green Rovers, again on loan for the duration of the 2021–22 season.[15]