He made his debut for Charlton Athletic on 31 August 2021 in a 6–1 EFL Trophy victory over Crawley Town.[4]
On the 29 April 2022, due to illness for Charlton number one Craig MacGillivray, Harness played the full 90 minutes of Charlton's last game of the season against Ipswich Town at Portman Road, with the Addicks losing 4–0.
On 13 May 2023, it was announced that Harness would leave the club when his contract expired in June.[5]
Billericay Town (loan)
Harness joined Billericay Town on a month's loan on 7 January 2020 where he made four appearances.[6][7]
Welling United (loan)
Harness joined Welling United on a loan on 8 December 2020 where he made two appearances.[8][9]
Dulwich Hamlet (loan)
On 22 January 2022, Harness joined Dulwich Hamlet on a one month loan deal.[10]
Bromley (loan)
On 25 October 2022, Harness joined Bromley on a 28-day loan.[11]
Harness was recalled by Charlton on 17 November 2022, having not made an appearance for Bromley.[12]
Milton Keynes Dons
On 20 June 2023, Harness agreed to join Milton Keynes Dons when his contract expired at Charlton Athletic on 30 June 2023.[13] He made his debut on 13 February 2024 in a 4–0 defeat to Bradford City.[14]
Gateshead (loan)
On 23 March 2024, Harness joined Gateshead of the National League on loan until the end of the 2023–24 season.[15]
Wealdstone (loan)
On 15 November 2024, Harness joined National League side Wealdstone on an initial one-month loan deal.[16]