[4] 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,[6] who had already been a teammate for several years during their development.
[7][8] 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.
[14][15] In May 2019, amid more goals and growing media praise for his performances,[16] 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.
[21] 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.
[29] 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.
[31] Turnbull finished the 2020–21 season with ten goals and won the PFA Scotland Young Player of the Year award,[32][33] proving to be a positive for Celtic in an otherwise disappointing campaign where they lost the Premiership title and failed to win a trophy.
[38] 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.
[43] Having never fully established himself in the Celtic starting lineup and with his contract due to expire in four months, on 1 February 2024 Turnbull signed for EFL Championship club Cardiff City on a permanent deal (the fee was undisclosed but reported at around £2 million) running to 2027.