Arriving home from Korea, Theseus disembarked all her aircraft and entered Portsmouth Dry Dock to undergo a refit. On the 8th June 1951, there was a change of command when Captain C. N. Lentaigne. DSC. RN. took over from Captain Bolt. The refit kept Theseus in Portsmouth Dockyard until mid September. After re-commissioning on the 13th September she put to sea to carry out engine, and flying trials. By the 19th September, Theseus was operating off the Cornish coast carrying out flying practice with Sea Furies of 807 Squadron, and Fireflies of 814 Squadron. She was then allocated to the Home Fleet as flagship of the 3rd Aircraft Carrier Squadron under the flag of Rear Admiral C. John, RN. On the 10th Oct she began an important convoy defence exercise which took place over a wide area of Scottish waters, the Atlantic, and the South Western Approaches, and lasted five days. Also taking part were the fleet carrier Indomitable, and twenty one destroyers and frigates. On board Theseus, as well as her own aircraft, were the Sea Furies from No 4 Squadron of the Royal Netherlands Navy.

14th October, the fleet set sail for Gibraltar, arriving on the 19th. During the time she was alongside, she operated a Hoverfly helicopter, but during the forenoon of 22nd October the machine crashed in Gibraltar Bay, killing its crew members.

During the whole of November, Theseus used Gibraltar as her base. Then on 2nd December she left to return to Portsmouth, arriving four days later. Whilst in Portsmouth dockyard, it was announced that, in February 1952 she was to be temporarily deployed to the Mediterranean Fleet to relieve HMS Ocean which was being prepared for service in Korea.

On 22nd January 1952 Theseus left Portsmouth Sound to set course for Gibraltar, and then Malta, arriving on 4th February. That morning the aircraft of 802 and 825 Squadrons were flown ashore, and during the forenoon the personnel were disembarked in Marsaxlokk Bay. Theseus then entered Grand Harbour. She put to sea on 11 February to land-on the Sea Furies of 807 Squadron, and Fireflies of 810 Squadron which had come from Ocean.

The last week of February saw Theseus at sea, taking part in fleet manoeuvres with the cruisers Glasgow and Liverpool, together with various escorts. On the 6th March there was a seven-day visit to Naples, and in early April she visited Tripoli. On the 19th April, having returned to Malta, she hoisted the flag of FO2 Mediterranean. Vice Admiral R.A.B. Edwards for an official visit to Greek ports. On 21st April, Theseus left Grand Harbour with other ships of the fleet, and set course for Piracus. During the passage an air defence exercise was carried out, and on the 23rd April the fleet anchored in Phaleron Bay, outside Piraeus, the port for Athens.

The visit was to mark the unveiling of a memorial to men of the Commonwealth Forces who had died in Greece during the Second World War. This was performed on the 25th April by King Paul of the Hellenes. The next day the King visited the fleet at anchor in the bay, and inspected the ship’s company on Theseus’ flight deck. The carrier left Phaleron Bay on 28th April, arriving back in Malta on the last day of the month.

During the first week of May, Theseus operated from Grand Harbour, but on the 27th May, with Glory having arrived in Malta from the Far East, she was able to begin her passage back to the UK, and she left for Gibraltar. On board for the journey were the personnel and aircraft of 812 and 814 Squadrons, who had completed a tour of duty in HMS Glory. Theseus left Gibraltar on 1st June, and arrived at Spithead three days later, before steaming up harbour to secure alongside South Railway Jetty on 5th June. Seven days later she was towed to No 14 dry dock for a nine-week assisted maintenance period. It was mid-August before she put to sea again.

Leaving Portsmouth on 18th August, Theseus landed on the aircraft of 804 Squadron before making her way north for exercises off Scotland. She was back in Portsmouth on 25th September. Five days later Theseus sailed for the Mediterranean once again to provide the necessary air cover, whilst Glory returned to Korean waters. She arrived in Grand Harbour on 9th October, and operated the Seafuries and Fireflies of 898 and 807 Squadrons.

On the last day of October she left Grand Harbour for a seven-day visit to the city of Trieste. The visit was followed by flying exercises in the Adriatic, but by 12th November Theseus was back at Malta. On Friday 28th November, she embarked the Duke of Edinburgh and left Grand Harbour to put on a display of dive bombing, cannon, and rocket firing, then anchored in Marsaxlokk Bay where the Duke of Edinburgh lowered his personal standard and left the carrier by launch. The display marked the end of Theseus’ second stint in the Mediterranean Fleet, and on 1st December, having embarked 802 and 825 Squadrons she left Malta to return, via Gibraltar, to Portsmouth. She arrived at Spithead on 9th December, and next day steamed up harbour to secure alongside Pitch House Jetty. On the 15th December, Captain D. Mcl Russell RN joined the ship and took over command the following day.

Christmas and New Year was spent at Portsmouth, and then Theseus put to sea again on 20th January 1953 to operate the Sea Furies of 812 and the Fireflies of 824 Squadrons. On 26th January 1953 she set course for Gibraltar, from where she operated for the whole of February, and the first two weeks of March.

Monday 16th March, having flown off her aircraft whilst steaming up Channel, Theseus returned to Portsmouth.

For Navy Days in the first week in April, an Attacker jet fighter was loaded on board and put on display on the flight deck. Then three weeks later Theseus put to sea again, and on 29th April she sailed to land-on the Sea Furies of 802 Squadron and the Fireflies of 824 Squadron for operations in the Channel until the end of May. There was a catapult failure on Monday 4th May whilst off the Isle of Wight. As a result, a Sea Fury ditched into the sea over the ship’s bow, fortunately the pilot was rescued.

In early June there was a visit to Torquay, which was followed by the Coronation Fleet Review at Spithead on the 15th June. After leaving Spithead on 16th June, Theseus steamed round to Milford Haven, and after embarking 802 and 824 Squadrons, she sailed for the Mediterranean. For this third short stint with the Mediterranean fleet, Theseus was to take the place of Ocean which had sailed east to relieve Glory off Korea.

Theseus arrived in Malta on 26th June, and 17 days later was ready to accompany the fleet on its summer cruise. The main port of call was Istanbul where, after passing units of the US Sixth Fleet which had just left the Turkish capital, the fleet made a majestic entry into the Bosporus watched by thousands of people gathered on both the Asian and European shores. Altogether 22 units of the fleet anchored off the city, and this huge presence let to protests from the Soviet Union that the earlier US visit, followed by the Mediterranean Fleets visit were tantamount to military demonstrations against the Soviet Union. However, the Turkish Government replied that ‘…the frequency of naval visits could only be interpreted as felicitous evidence of the friendly ties binding Turkey to the countries of which these fleets belong.’ The visit ended on 3rd August when the fleet left Istanbul to carry out exercises in the Aegean. Theseus then returned to Malta on 6th August to carry out maintenance.

Thursday 27th August saw Theseus leave Grand Harbour to land-on her aircraft and carry out flying practice in local waters. Then on 9th September she set sail for Phaleron Bay, Athens, arriving later the same day. Next day shore leave was granted, and there was a regular stream of ‘liberty men’ leaving for a trip into Piraeus. However, that morning, at the western end of the island of Cyprus, within twenty miles of the town of Paphos, a severe earthquake had rocked the area killing over 40 people and injuring hundreds more, leaving at least 50,000 without any food, water or accommodation. Although British Army units on the island had joined the rescue teams, it was clear that more help was urgently needed, and on 10th September Theseus was ordered to sail for Paphos as soon as possible. All shore leave was immediately cancelled and additional embarkation patrols were landed to round-up all liberty men. Finally, just after midnight on 11th September, with everyone back on board, Theseus weighed anchor and set course for Paphos Bay.

Volunteers were called for to help with the work ashore, and two Dragonfly helicopters used Theseus as a base for two days, flying some 136 sorties, carrying food, tents, and injured people. The working parties from the ship did a magnificent job pitching tents and distributing food to over 4,000 villagers. Finally, during the evening of the 14th September Theseus weighed anchor to return to Malta, arriving in Grand Harbour on the 17th.

Further exercises took place and she was back in Malta by 16th October. Theseus’ time with the Mediterranean Fleet, and her career as a fixed-wing aircraft carrier were drawing to a close, and on 20th October she left Malta to return home to Portsmouth. She made a three day stop at Gibraltar, and on the 29th October she carried out her final fixed-wing flying operations when her aircraft were launched to their bases ashore. That evening she anchored at Spithead, and the following forenoon she steamed up harbour to secure alongside Pitch House Jetty.

By mid November the ship’s company had been much reduced, and on 20th January 1954 she was taken into dockyard hands for a refit which would convert her for use as a training ship with the Home Fleet.

Theseus spent the first few months of 1954 in No 14 dry dock, during which time classrooms were built in the main hangar which was also to double up as a wet-weather parade ground. Her mess decks were also adapted to accommodate the various types of trainees who would go to sea for the first time. In mid May, she was moved out of dry dock, and on Monday 12 July Theseus sailed from Portsmouth to carry out trials. After returning for further adjustments, she left for Portland, securing to a buoy in the harbour.

Theseus was replacing Implacable on the Home Fleet Training Squadron, and during the following weeks, men under training and stores were transferred from Implacable. Then on Tuesday 17th August, Captain H.N.S. Brown. CBE. RN. also transferred to Theseus and officially took command. Next day the Flag Officer Training Squadron, Rear Admiral Carlill hoisted his flag in Theseus, which was to remain the Squadron’s flagship until the end of her career.