Jack had a loan spell during 2023-2024 season with Greenock Morton where he suffered an injury.
[5] In November 2021, he signed a contract extension with the Gers, keeping him at the club until the summer of 2023.
[7][8] On 29 September 2023, Harkness signed for Greenock Morton on a season-long loan deal.
[9] On 1 August 2024, he joined League Two side Stirling Albion on a season long loan.
[10] Harkness made his international debut for Scotland U16 on 22 January 2020 in a 3–2 defeat to Australia.