Two ships were originally planned, but only one carrier was included in the 1938 Naval Programme as the other was delayed a year by the government. While under construction, the ships had their forward lift enlarged to take non-folding aircraft like the Hawker Sea Hurricane and the early models of the Supermarine Seafire, the flight deck was widened abreast the forward lift, splinter protection was added as were seven diesel generators, each in their own watertight compartment. All these changes increased the ships' displacement by 540 long tons (550 t) at deep load.
Implacable?'?s construction was suspended in 1940 in favour of escorts needed in the Battle of the Atlantic so that the two carriers were launched within days of each other. She embarked portions of her air wing for training in late August and was assigned to the Home Fleet on 7 October at Scapa Flow after working up. She joined ships searching for the Tirpitz a week later and some of her Fairey Fireflies spotted the battleship off Håkøya Island near Tromsø. Subsequently her Fireflies successfully attacked targets in Norway. In late October, the carrier's Seafires arrived and she participated in Operation Athletic off the Norwegian coast, sinking four warships and two merchant ships and damaging a German submarine. In November and December, Implacable provided air cover for minelaying operations and attacked German shipping off the Norwegian coast. On 15 December she began a refit at Rosyth preparatory to her transfer to the BPF, which included augmenting her light AA armament.
When the refit was completed on 10 March 1945, the ship embarked an enlarged air wing with 81 aircraft (48 Seafires, 12 Fireflies, and 21 Grumman TBF Avenger torpedo bombers), the largest number of aircraft aboard a British carrier up to that time. Implacable arrived at Sydney, Australia in May and joined the other carriers of the 1st Aircraft Carrier Squadron (1st ACS) at Manus Island after their return from the invasion of Okinawa in June. On 14–15 June, the carrier attacked the Japanese naval base at Truk. After working up with the other carriers, the ship sailed with the 1st ACS on 6 July to rendezvous with the American carriers of Task Force 38 off the Japanese home island of Honshu ten days later. The British carriers began flying sorties against Japanese targets on Honshu on 17 July and Implacable?'?s aircraft, before departing the area on 11 August to replenish, flew over 1,000 sorties. She arrived at Sydney on 24 August and spent most of the rest of the year ferrying Allied prisoners of war and soldiers back to Australia and Canada. In January 1946, together with her sister Indefatigable and several other ships, she made a number of port visits in Australia and New Zealand. The ship was refitted in Sydney in preparation for her return home on 3 June where she became the deck-landing training carrier for the Home Fleet.
Implacable temporarily became a trials carrier in October 1947 as her own air group was not yet ready and she was refitted from October to December 1948 in preparation of service as the Home Fleet flagship. She embarked a squadron each of de Havilland Sea Hornets and Blackburn Firebrands in April 1949 and became the flagship of Admiral Sir Philip Vian on 29 April. A squadron of de Havilland Sea Vampires flew from her deck later that year and her air group was augmented by a squadron of Fairey Barracudas in 1950. Implacable was placed in reserve in September 1950 and slowly converted into a training ship by the addition of extra accommodation and classrooms, including the addition of a deckhouse on her flight deck. She was recommissioned on January 1952 as the flagship of the Home Fleet Training Squadron. Together with Indefatigable, she was present during the Coronation Fleet Review of Queen Elizabeth II on 15 June 1953. Four months later, Implacable ferried a battalion of troops from Plymouth to Trinidad in response to a crisis in British Guiana. She was decommissioned on 1 September 1954 and sold for breaking up on 27 October 1955.