 Cassidy worked as planning officer in the Ministry of Development of Northern Ireland from 1967 to 1968 and in the Department of Lands and Settlements, Kenya from 1968 to 1970.From 1972 to 1975 he was curate of Christ Church, Clifton, from 1975 to 1982 the vicar of St Edyth's, Sea Mills and of St Paul's, Portman Square, London from 1982 to 1987.From 1987 to 1999, he was an archdeacon and a canon residentiary of St Paul's Cathedral.