On 14 July 2019, Cassidy was at corner forward for the Ulster under-20 final against Tyrone. Cassidy scored a point but was forced off injured after 13 minutes. Tyrone went on to win the match by 4–13 to 1–10.[6]
Senior
Cassidy joined the Derry senior team in 2020, making his National League debut on 17 October, scoring 1–1 after coming on as a late substitute against Longford.[7] Cassidy made his championship debut on 1 November, coming on as a substitute in an Ulster-quarter final loss to Armagh.[8]
On 19 June 2021, Derry faced Offaly in the Division 3 league final. Cassidy was brought on as a second-half substitute as Derry were 0–21 to 1–6 winners.[9] On 11 July, Cassidy made his first championship start against Donegal in the Ulster quarter-final. Derry lost the match by a point, with Donegal taking the lead for the first time in injury time.[10]
On 1 May 2022, Cassidy was in the half-forward line as Derry beat reigning All-Ireland champions Tyrone by eleven points in the Ulster quarter-final.[11] Derry went on to reach their first Ulster final in eleven years with a win over Monaghan, with Cassidy scoring two points.[12] The Ulster final took place on 29 May, with Derry facing Donegal. Cassidy scored a point as Derry claimed an extra-time victory to win their first provincial title since 1998.[13] Cassidy scored 1–1 in the All-Ireland quarter final win over Clare, as Derry set up a semi-final against Galway.[14] Derry's season ended at the semi-final stage as Galway were five-point winners.[15]
On 21 January 2023, Cassidy scored a point and was named man of the match as Derry won the Dr McKenna Cup, beating Tyrone in the final.[16][17] Derry secured promotion to Division 1 after winning their first six games,[18] and went on to face Dublin in the Division 2 final on 2 April. Cassidy scored two points, but Dublin won the match by 4–6 to 0–11.[19] In the Ulster Championship, Cassidy scored 1–2 against Fermanagh,[20] and two points against Monaghan as Derry reached back-to-back provincial finals.[21] In the Ulster final on 14 May, Cassidy had his penalty saved in the shoot-out, but Derry still beat Armagh, defending the Ulster title.[22] On 16 July, Cassidy scored two points as Derry lost the All-Ireland semi-final to defending champions Kerry.[23] At the end of the season, Cassidy was nominated for an All-Star award for the first time.[24]
In the 2024 league, Derry topped the table, qualifying for the final against Dublin.[25] Cassidy started the final at wing forward, and Derry won the league for the first time since 2008 after a penalty shoot-out win.[26]