He played with, and later managed, club side Bellaghy and the Derry senior football team.
He was centre-back on Bellaghy's first-ever Derry SFC-winning team in 1956, and was also part of the club's four-in-a-row success from 1958 to 1961.
[1] Cassidy first appeared on the inter-county scene as a member of the Derry senior football team.
He was part of the team's Ulster SFC-winning campaign in 1958 and was also an unused substitute when Derry were beaten by Dublin in the 1958 All-Ireland final.
[3] Cassidy's success at club level resulted in two subsequent stints as Derry manager.