Motherwell

Raised in Wishaw where he attended Coltness High School,^[5] Turnbull joined the Motherwell Youth Academy in 2009.^[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.^[9][8]

Turnbull made his senior debut for Motherwell on 10 February 2018, in a 2–0 victory away at Dundee in the Scottish Cup.^[10][11] 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.^[7][12][13] A few days later he signed a new contract with Motherwell, running until summer 2021.^[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.^[17] Turnbull won the SFWA Young Player of the Year award for 2018–19, voted for by Scottish football journalists.^[18] He was the first Motherwell player to win that award since James McFadden in 2001–02.^[18]

In June 2019, Motherwell agreed a club-record £3.25 million fee with Celtic for Turnbull.^[19][20] 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.^[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.^[22] 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.^[20] He made his return to the first team as a second half substitute at home to St Mirren on 25 February.^[23] On 11 March 2020, Turnbull signed a contract extension with Motherwell to run until 2022.^[24] 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.^[25]

Celtic

Turnbull joined Celtic on 27 August 2020, on a four-year deal.^[26] The fee of around £3 million^[27] was a club record for Motherwell.^[28]

After three substitute appearances during September, he made his first start for the club on 4 October against St Johnstone.^[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. 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.^[30] His subsequent form in the Scottish Premiership saw him named the SPFL Player of the Month for December.^[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.^[34][35][36]

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.^[37] Later that month, he scored a hat-trick in a 6–0 victory against St Mirren in the league.^[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.^[39][40]

On 3 September 2022, Turnbull scored in a 4–0 win against Rangers, taking advantage of an error from Jon McLaughlin.^[41] He was an unused substitute in that season's Scottish League Cup final^[42] and came off the bench in the 2023 Scottish Cup final.^[43]

Cardiff City

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.^[44][45]