During the Second World War, Synnot served aboard the destroyer in the Battle of Cape Matapan, for which he was mentioned in despatches, and during the evacuation of Greece and Crete. With the Royal Navy, he saw service on the battleship and was on board the destroyer when she sank off Iceland in 1942 after being accidentally rammed by the battleship. Subsequently, Synnot served for two years on the Australian destroyer on North Sea convoy duty and during the North Africa landings, eventually becoming the ship's executive officer. In 1945, Synnot qualified as a gunnery officer and served on the staff of gunnery schools in Australia. Promoted to commander in 1954, he took charge of in 1956. He became captain of the Daring-class destroyer in 1960. In 1950, Synnot had taken part in the Bridgeford Mission to Malaya, which advised the Australian government on the Malayan Emergency. His report on the options for providing naval support for the British laid the foundations for Australian naval involvement in the region and led to Synnot's secondment to command the Royal Malaysian Navy from 1962 to 1965. On his return to Australia, Synnot attended administrative staff college before returning to sea in 1966 as Captain of the Australian aircraft carrier HMAS Sydney, then in 1967, the carrier HMAS Melbourne. He was the only officer to command both aircraft carriers. After a year at the Imperial Defence College in London, he returned to Australia as director general of fighting equipment. Promoted to rear-admiral in 1970, he became chief of naval personnel and subsequently deputy chief of naval staff. He became Flag Officer Commanding HM Australian Fleet in 1973. In 1974, he was appointed director joint staff in the Australian Defence Department, and played a leading role in the relief effort following the devastation of Darwin by Cyclone Tracy. In 1976, Synnot was promoted to vice admiral and appointed Chief of Naval Staff. He initiated a review of the Navy Office and of the Navy's structure of command and control. He drew up a blueprint for the maintenance of naval capability into the future, and oversaw the Navy's guided-missile frigate project. Extremely able and practical, Synnot came to be regarded as one of the country's most outstanding defence force chiefs. A strong believer in deterrence and an advocate of close co-operation with America and countries in the Pacific region, Synnot emphasised the need for a strong military capability for national defence and for joint operations with Australia's allies overseas. He was said to have done more to equip Australia's armed forces with up-to-date military technology than any of his predecessors. In particular, he was instrumental in persuading the Australian government of the need to upgrade the country's air force with the acquisition of the F/A-18 Hornet. He was also behind the decision to acquire the British aircraft carrier as a replacement for the ageing HMAS Melbourne. However, Britain withdrew the offer to sell Invincible after the Falklands War. Synnot retired on 20 April 1982.
