Campbell made his professional debut for Aberdeen as a late substitute in a league match against Celtic on 12 May 2017, aged 16 years, 1 month, 23 days. In doing so he became the youngest ever Aberdeen player, breaking a record previously held by Jack Grimmer, and also became the first player from the Scottish Football Association's performance school programme to make their senior competitive debut. Earlier the same day he sat an English exam at his school, Hazlehead Academy. A few days prior to his debut, Campbell had been named as the club's Academy 'Player of the year' for the third year in succession. Campbell's second league appearance came one year and one day after his first, against the same opponents Celtic; again he was introduced as a substitute, and played his part in a 1–0 win for his team which secured runners-up spot on the last day of the season. On 10 September 2018 Campbell signed a contract extension, keeping him at the club until Summer 2021. He scored his first professional goal on 29 December 2018, the second in a 2–1 away win at Livingston in the Scottish Premiership. On 12 March 2019, while still 17 years old, Campbell made his first start for Aberdeen in a 2–0 Scottish Cup quarter-final replay victory against Rangers at Ibrox Stadium. On 3 April 2019, two weeks after his 18th birthday, he made his first league start for the Dons in a 3–1 Premiership win against Motherwell. Campbell signed a new three-and-a-half-year contract with Aberdeen on 13 December 2019. At the end of the season it was announced that he had won the club's Young Player of the Year award for season 2019–20.
International career
Campbell was a member of the Scotland under-16 squad for the 2016 Victory Shield tournament. On 13 February 2018, Campbell scored twice as Scotland under-17s beat Spain 2–1 in Marbella, the first Scottish national team to win in Spain since the full team did so in 1963. On 10 October 2018, Campbell captained Scotland under-18s as they drew 1–1 with Uzbekistan in a friendly international. Campbell was named in the Scotland under-19s squad for the first round of qualifiers for the Euro 2019 U19 championships and made his debut in a 5–0 win against San Marino on 17 November 2018, and was a second-half substitute in a 2–2 draw against Sweden 3 days later, a result which guaranteed Scotland topped their section and progression to the elite round of qualification, eventually missing out, losing out in the final game to defending champions Portugal. He was named in the Under 19 squad for the first round of qualifiers for the Euro 2020 U19 championships, starting all three games, including a 1–0 victory over Germany. This result meant Scotland again topped their group and progressed to the elite qualification round. On 11 November 2019, he received his first call up to the Scotland under-21s for a Euro 2021 qualifier against Greece
