The vessel remained unfinished until 1957; then she entered service on 18 November 1959 as HMS Hermes after extensive modifications which included installation of a massive Type 984 'searchlight' 3D radar, a fully angled deck with a deck-edge lift, and steam catapults.

Hermes initially operated Supermarine Scimitar, de Havilland Sea Vixen, and Fairey Gannet fixed-wing aircraft, together with Westland Whirlwind helicopters.

[2] On 16 November 1962, Hermes was cruising off of the Pembrokeshire coast in Wales when one of her helicopters carrying two Members of Parliament, Lord Windlesham and the MP for Loughborough, John Cronin, back from the carrier, which they had been visiting, to RNAS Brawdy, crashed off St David's Head.

"[5] John Hay, Civil Lord of the Admiralty, said in Parliament on 2 March 1964 that "Phantoms will be operated from "Hermes", "Eagle" and the new carrier when it is built.

"[6] This seemed optimistic, as most sources believed Victorious was the smallest carrier then in commission that the modified RN F-4K versions of the Phantom could realistically have operated from.

[7] Since this lower weight at launch from Hermes would be achieved by carrying less fuel, combat air patrol duration would be 25 to 50 percent less than from Eagle; from 2–2.5 hours to 1–1.5 hrs,[8] and only partly compensated by refuelling when airborne.

It was optimistically believed Hermes could replace its Vixens with Spey-powered Phantoms on a one-to-one basis,[9] i.e. 11–12 with 7–8 Blackburn Buccaneer strike aircraft.

While the Phantoms built for the RN were modified in ways similar to Vought F-8 Crusaders for the French Navy – improving deceleration on landing – the modifications were not entirely successful.

Hermes's flight deck was too short, her arresting gear as well as her catapults were not powerful enough to recover or launch the F-4Ks, even though they were slightly lighter, more economical and higher performing than their US Navy counterparts[citation needed].

By 1976, with the Soviet submarine threat becoming apparent and through NATO pressure, a further mild conversion was performed for Hermes to become an anti-submarine warfare carrier to patrol the North Atlantic.

Hermes underwent one more conversion and new capabilities were added when she was refitted at Portsmouth from 1980 to June 1981, during which a 12° ski-jump and facilities for operating BAe Sea Harriers were incorporated.

[15] After this refit the air wing comprised: Hermes was due to be decommissioned in 1982 after the 1981 Defence Review (that would have made the Royal Navy considerably smaller) by the British government, but when the Falklands War broke out, she was made the flagship of the British forces, setting sail for the South Atlantic just three days after the Argentine invasion of the Falkland Islands.

As she was the RN's largest carrier, she was considered too valuable to risk in close to the Falklands, due to the possibility of Argentine air force attacks.

In 1983, when the proposed sale of the aircraft carrier Invincible to the Royal Australian Navy was cancelled following the Falklands War, an offer was made to sell Hermes and a squadron of Sea Harriers to Australia.