Celtic Aitchison graduated from
Celtic's school of excellence at
St Ninian's High School in
Kirkintilloch in February 2016, at which time he joined Celtic full-time and became part of their
Under 17 squad. He played in the Celtic U17 side that defeated their Rangers counterparts 4–0 in the
Glasgow Cup final in April 2016. At this time, he also signed a three-year contract. During his youth career he also appeared and scored in the
2015–16 and
2016–17 editions of the
UEFA Youth League, 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. 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. Aitchison joined
League One club
Dumbarton on loan in August 2018. He was then loaned to
Championship club
Alloa Athletic in January 2019. On 2 September 2019, Aitchison moved on a season-long loan deal to English
League Two club
Forest Green Rovers. He scored on his debut as a substitute in a 1–0 win away to
Cambridge United on 7 September 2019.
Barnsley On 5 October 2020, Aitchison joined
Championship side
Barnsley on a three-year contract after leaving Celtic.
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.
Motherwell On 31 January 2023, Aitchison joined
Scottish Premiership club
Motherwell on a permanent deal until the end of the season.
Exeter City On 28 June 2023, Aitchison agreed to join League One side
Exeter City on a two-year deal. In May 2025, Aitchison signed a new one-year deal until the end of the 2025/26 season. ==Career statistics==