Motherwell Raised in
Wishaw where he attended
Coltness High School, Turnbull joined the
Motherwell Youth Academy in 2009. He was selected for the Scotland football team (represented by
North Lanarkshire school pupils) at the 2014
International Children's Games alongside fellow Motherwell player
Jake Hastie, who had already been a teammate for several years during their development. On 27 April 2016, Turnbull, Hastie and another long-term academy colleague
Allan Campbell were in the
Well team that won the
Scottish Youth Cup, beating
Heart of Midlothian 5–2. On 8 May 2018, he made his first start and
Scottish Professional Football League debut in a 1–0 victory away to
Partick Thistle. Later that month he was an unused substitute in the
2018 Scottish Cup Final which Motherwell lost 2–0 to
Celtic. On 31 October 2018, Turnbull scored his first goal for Motherwell against
St Mirren. In January 2019, he was the sole goalscorer in two consecutive league wins in the space of four days, at home to
Hibernian and away to Dundee. A few days later he signed a new contract with Motherwell, running until summer 2021. In May 2019, amid more goals and growing media praise for his performances, he was nominated for the season's
PFA Scotland Young Player of the Year, along with Jake Hastie; the award was won by
Ryan Kent. Turnbull won the
SFWA Young Player of the Year award for 2018–19, voted for by Scottish football journalists. He was the first Motherwell player to win that award since
James McFadden in 2001–02. The proposed deal collapsed when medical tests found that Turnbull would require preventative surgery on his left knee, and the clubs could not agree revised terms. Turnbull returned to Motherwell, had his knee operation – performed by renowned
London surgeon
Andy Williams – then underwent a rehabilitation programme across the winter (alongside teammate
Charles Dunne who was recovering from a groin injury) and resumed full training in February 2020. His mental and physical recovery was chartered in a behind-the-scenes club documentary which followed him from the day of his collapsed move through operations, rehabilitation and to his eventual return to first-team action. On 11 March 2020, Turnbull signed a contract extension with Motherwell to run until 2022. With the 2019–20 season halted days afterwards due to the
COVID-19 pandemic in Scotland, he successfully re-established himself as an important member of the side at the outset of
2020–21.
Celtic Turnbull joined
Celtic on 27 August 2020, on a four-year deal. The fee of around £3 million was a club record for Motherwell. While there was initially few opportunities in the early part of the season, on 10 December 2020 Turnbull started in a
UEFA Europa League home fixture against
Ligue 1 side
Lille OSC. He scored his first goal for Celtic and made an assist for
Christopher Jullien in a 3–2 win against the French champion in the last group stage match that season. His subsequent form in the
Scottish Premiership saw him named the SPFL Player of the Month for December. Turnbull finished the 2020–21 season with ten goals and won the
PFA Scotland Young Player of the Year award, proving to be a positive for Celtic in an otherwise disappointing campaign where they lost the Premiership title and failed to win a trophy. Under new Celtic manager
Ange Postecoglou, Turnbull played regularly in the first half of the 2021–22 season. On 12 August 2021, he scored twice in a 3–0 (agg 7–2)
UEFA Europa League play-off win against
Czech First League side
FK Jablonec. Later that month, he scored a
hat-trick in a 6–0 victory against
St Mirren in the league. Turnbull featured as a starter in the
2021–22 Scottish League Cup final but was forced off with a
hamstring injury after 27 minutes; Celtic defeated
Hibernian 2–1 and he collected a winner's medal. He was an unused substitute in that season's
Scottish League Cup final Turnbull scored his first goal for the Bluebirds in a 3–1 defeat to
Blackburn Rovers on 9 November 2024. On 16 December 2025, he scored the only goal for his club in a 3–1 home defeat to
Chelsea in the quarterfinals of the
EFL Cup. ==International career==