Cassidi joined the
Royal Navy in 1938. Commissioned in 1943, he was promoted to
sub-lieutenant in February 1944. He served in and with the Iceland Patrols and Russian Convoys during the
Second World War and also took part in the
Normandy landings. and
lieutenant commander in October 1953. He became the
commanding officer of
820 Naval Air Squadron in 1954. Promoted to
commander on 31 December 1956, he became the commanding officer of in 1960. He became
Director-General, Naval Manpower and Training in 1975 and
Flag Officer Naval Air Command in 1978. He then became
Second Sea Lord and Chief of Naval Personnel in 1979 and
Commander-in-Chief Naval Home Command in 1982. He retired in 1985. ==Family and later life==