Her first assignment after completion and working up was with the Mediterranean Fleet, in which her aircraft's most notable achievement was sinking one Italian battleship and badly damaging two others during the Battle of Taranto in late 1940.

After sustaining damage on the voyage home in late 1941 by a collision with her sister ship Formidable, Illustrious was sent to the Indian Ocean in early 1942 to support the invasion of Vichy French Madagascar (Operation Ironclad).

Her aircraft attacked several targets in the Japanese-occupied Dutch East Indies over the following year before Illustrious was transferred to the newly formed British Pacific Fleet (BPF).

Admiral Sir Reginald Henderson, Third Sea Lord and Controller of the Navy, was determined not to simply modify the previous unarmoured Ark Royal design.

[10] The main armament of the Illustrious class consisted of sixteen quick-firing (QF) 4.5-inch (110 mm) dual-purpose guns in eight twin-gun turrets, four in sponsons on each side of the hull.

[7] A year later, in preparation for her service against the Japanese in the Pacific, one starboard octuple "pom-pom" mount, directly abaft the island, was replaced by two 40 mm Bofors AA guns.

[7] By 1945, accumulated wear-and-tear as well as undiagnosed shock damage to Illustrious's machinery caused severe vibrations in her centre propeller shaft at high speeds.

[22] Construction was delayed by slow deliveries of her armour plates because the industry had been crippled by a lack of orders over the last 15 years as a result of the Washington Naval Treaty.

The ship was docked in Clydeside for a minor refit the following day; she arrived in Scapa Flow on 15 August, and became the flagship of Rear Admiral Lumley Lyster.

[33] She sailed from Alexandria on 6 November, escorted by the battleships Warspite, Malaya, and Valiant, two light cruisers, and 13 destroyers, to provide air cover for another convoy to Malta.

The Royal Air Force (RAF) had positioned a Short Sunderland flying boat off the harbour to search for any movement to or from the port and this was detected at 17:55 by acoustic locators and again at 20:40, alerting the defenders.

The two airstrikes had changed the balance of power in the Mediterranean by rendering the Conte di Cavour unavailable for the rest of the war, and badly damaging the Littorio and the Duilio.

One bomb penetrated the outer edge of the forward port flight deck and detonated about 10 feet (3.0 m) above the water, riddling the adjacent hull structure with holes which caused flooding in some compartments and starting a fire.

One additional Swordfish, piloted by Lieutenant Charles Lamb, was attempting to land when the bombs began to strike and was forced to ditch when it ran out of fuel; the crew was rescued by the destroyer Juno.

Illustrious was not struck during these attacks but was near-missed several times and the resulting shock waves from their detonations dislodged enough hull plating to cause an immediate 5-degree list, cracked the cast-iron foundations of her port turbine, and damaged other machinery.

Neither ship was seriously damaged, but Illustrious had to reduce speed to shore up sprung bulkheads in the bow and conduct temporary repairs to the forward flight deck.

[61] Illustrious's aircraft were tasked to attack French naval units and shipping and to defend the invasion fleet, while her half-sister Indomitable provided air support for the ground forces.

The next morning, Martlets from 881 Squadron intercepted three Potez 63.11 reconnaissance bombers, shooting down two and forcing the other to retreat, while Swordfish dropped dummy parachutists as a diversion.

On 26 February she began a refit at Birkenhead that lasted until 7 June during which her flight deck was extended, new radars were installed, her light anti-aircraft armament was augmented, and two new arrestor wires were fitted aft of the rear lift which increased her effective landing area.

The British ships were spotted by Blohm & Voss BV 138 flying boats and 890 Squadron shot down two of them before the fleet returned to Scapa Flow on 29 July.

She transferred to Greenock at the end of the month and sailed on 5 August to provide air cover for the ocean liner RMS Queen Mary as she conveyed Prime Minister Winston Churchill to the Quebec Conference.

The fleet departed Trincomalee on 21 March to rendezvous with the American carrier Saratoga in preparation for combined operations against the Japanese facilities in the Dutch East Indies and the Andaman Islands.

As the fleet was withdrawing, Illustrious's CAP intercepted and shot down a Nakajima Ki-43 (codenamed "Oscar") fighter and a Mitsubishi Ki-21 "Sally" medium bomber on reconnaissance missions.

[80] She arrived on 10 February and repairs began when she entered the Captain Cook Dock in the Garden Island Dockyard the next day, well before it was officially opened by the Duke of Gloucester, the Governor-General of Australia on 24 March.

[82] From 26 March to 9 April, the BPF attacked the airfields with each two-day period of flying operations followed by two or three days required to replenish fuel, ammunition and other supplies.

The light AA guns managed to sever its port wing so that it missed the ship, although its starboard wingtip shattered the Type 272's radome mounted on the front of the bridge.

[90] Illustrious resumed her duties in early 1949 and conducted trials and training for Avengers, Fireflies, Gloster Meteors, de Havilland Sea Hornets, Vampires and Seafires.

[92] After a brief refit in early 1952, she participated in Exercise Castanets off the Scottish coast in June and hosted 22,000 visitors during Navy Days at Devonport Royal Dockyard in August.

Jellicoe was relieved by Captain R. D. Watson on 26 September and Illustrious resumed training until 9 December when her crew was granted leave and the ship began a refit.

The ship resumed flying training off the north coast of Scotland in October for two weeks, but she spent most of the rest of the year on trials of the new mirror-landing system that automated the process of landing aircraft aboard.