Diack made his Partick Thistle debut coming off the bench in a Scottish Championship game against Hamilton Academical in January 2023.
[9] Diack scored his first goal of the 2024–25 season in a 6–0 away win over Edinburgh City FC in the Scottish League Cup.
[10] Diack was involved in a referee admin error during the final game of the Scottish League Cup group stages in a 0–0 draw away to Motherwell, in which Thistle manager Kris Doolan attempted to substitute the striker on, but was denied the substitution, due to an admin error in which the matchday referee had accidentally cropped Diack's name out of Thistle's matchday squad, meaning he was not eligible to play, despite the club submitting his name.
This incident became a source of controversy in the media over officials handling of confirming matchday squads.
[11][12][13] On the 14th of January 2025 Diack joined fellow Scottish Championship side Airdrie on loan until the end of the season.