The main document was a personal letter from Lieutenant General Sir Archibald Nye, the vice chief of the Imperial General Staff – who had a deep knowledge of ongoing military operations – to General Sir Harold Alexander, commander of the Anglo-American 18th Army Group in Algeria and Tunisia under General Eisenhower. After several attempts at drafting the document did not generate something that was considered natural, it was suggested that Nye should draw up the letter himself to cover the required points. The letter covered several purportedly sensitive subjects, such as the (unwanted) award of Purple Heart medals by US forces to British servicemen serving with them and the appointment of a new commander of the Brigade of Guards. Montagu thought the result was "quite brilliant"; the key part of the letter stated that

We have recent information that the Boche [the Germans] have been reinforcing and strengthening their defences in Greece and Crete and C.I.G.S. [Chief of the Imperial General Staff] felt that our forces for the assault were insufficient. It was agreed by the Chiefs of Staff that the 5th Division should be reinforced by one Brigade Group for the assault on the beach south of CAPE ARAXOS and that a similar reinforcement should be made for the 56th Division at KALAMATA.

The letter went on to identify Sicily and the Dodecanese as "cover targets" for the assaults, along with justifications for their selection.

There was also a letter of introduction for Martin from his putative commanding officer, Vice-Admiral Lord Louis Mountbatten, the chief of Combined Operations, to Admiral of the Fleet Sir Andrew Cunningham, the commander-in-chief Mediterranean Fleet and Allied naval commander in the Mediterranean. Martin was referred to in the letter as an amphibious warfare expert on loan until "the assault is over". The document included a clumsy joke about sardines, which Montagu inserted in the hope that the Germans would see it as a reference to a planned invasion of Sardinia. A single black eyelash was placed within the letter to check if the Germans or Spanish had opened it.

Montagu considered that there would be a possible "Roman Catholic prejudice against tampering with corpses", which could miss the documents stored in the corpse's pockets, so they added them to an official briefcase that would not be overlooked. To justify carrying documents in a briefcase, Major Martin was given two proof copies of the official pamphlet on combined operations written by the author Hilary Saunders – then on Mountbatten's staff – and a letter from Mountbatten to Eisenhower, asking him to write a brief foreword for the pamphlet's US edition. The planning team first thought of having the handle clutched in the corpse's hand, held in place by rigor mortis, but the rigor would probably wear off and the briefcase would drift away. They therefore equipped Martin with a leather-covered chain, such as was used by bank and jewellery couriers to secure their cases against snatching. The chain unobtrusively runs down a sleeve to the case. To Montagu it seemed unlikely that the major would keep the bag secured to his wrist during the long flight from Britain, so the chain was looped around the belt of his trench coat.

Montagu and Cholmondeley gave consideration to the location of the corpse's delivery. It had long been assumed by the pair that the western coast of Spain would be the ideal location. Early in the planning they investigated the possibility of Portuguese and French coasts, but rejected those in favour of Huelva on the coast of southern Spain, after advice was taken from the Hydrographer of the Navy regarding the tides and currents best suited to ensure the body landed where it was wanted. Montagu later outlined that the choice of Huelva was also made because "there was a very active German agent ... who had excellent contacts with certain Spaniards, both officials and others". The agent – Adolf Clauss, a member of the Abwehr – was the son of the German consul, and operated under the cover of an agriculture technician; he was an efficient and effective operative. Huelva was also chosen because the British vice-consul in the city, Francis Haselden, was "a reliable and helpful man" who could be relied upon, according to Montagu.

The body was supposed to be the victim of an aeroplane crash, and it was decided that to try to simulate the accident at sea using flares and other devices could be too risky and open to discovery. After seaplanes and surface ships were dismissed as being problematic, a submarine was chosen as the method of delivering the corpse to the region. To transport the body by submarine, it needed to be contained within the body of the boat, as any externally mounted container would have to be built with a skin so thick it would alter the level of the waterline. The canister needed to remain airtight and keep the corpse as fresh as possible through its journey. Spilsbury provided the medical requirements and Cholmondeley contacted Charles Fraser-Smith of the Ministry of Supply to produce the container, which was labelled "Handle with care: optical instruments".

On 13 April 1943 the committee of the Chiefs of Staff met and agreed that they thought the plan should proceed. The committee informed Colonel John Bevan – the head of London Controlling Section, which controlled the planning and co-ordination of deception operations – that he needed to obtain final approval from Churchill. Two days later Bevan met the prime minister – who was in bed, wearing a dressing gown and smoking a cigar – in his rooms at the Cabinet War offices and explained the plan. He warned Churchill that there were several aspects that could go wrong, including that the Spaniards might pass the corpse back to the British, with the papers unread. Churchill replied that "in that case we shall have to get the body back and give it another swim". Churchill gave his approval to the operation, but delegated the final confirmation to Eisenhower, the overall military commander in the Mediterranean, whose plan to invade Sicily would be affected. Bevan sent an encrypted telegram to Eisenhower's headquarters in Algeria requesting final confirmation, which was received on 17 April.

In the early hours of 17 April 1943 the corpse of Michael was dressed as Martin, although there was one last-minute hitch: the feet had frozen. Purchase, Montagu and Cholmondeley could not put the boots on, so an electric heater was located and the feet defrosted enough to put the boots on properly. The pocket litter was placed on the body, and the briefcase attached. The body was placed in the canister, which was filled with 21 pounds (9.5 kg) of dry ice and sealed up. When the dry ice sublimated, it filled the canister with carbon dioxide and drove out any oxygen, thus preserving the body without refrigeration. The canister was placed in the 1937 Fordson van of an MI5 driver, St John "Jock" Horsfall, who had been a racing champion before the war. Cholmondeley and Montagu travelled in the back of the van, which drove through the night to Greenock, west Scotland, where the canister was taken on board the submarine HMS Seraph, which was preparing for a deployment to the Mediterranean. Seraph's commander, Lt. Bill Jewell, and crew had previous special operations experience. Jewell told his men that the canister contained a top secret meteorological device to be deployed near Spain.

On 19 April Seraph set sail and arrived just off the coast of Huelva on 29 April after having been bombed twice en route. After spending the day reconnoitring the coastline, at 4:15 am on 30 April, Seraph surfaced. Jewell had the canister brought up on deck, then sent all his crew below except the officers. They opened the container and lowered the body into the water. Jewell read Psalm 39 and ordered the engines to full astern; the wash from the screws pushed the corpse toward the shore. The canister was reloaded and the submarine travelled 12 miles (19 km) out where it surfaced and the empty container was pushed into the water. As it floated, it was riddled with machine gun fire so that it would sink. Because of the air trapped in the insulation, this effort failed, and the canister was destroyed with plastic explosives. Jewell afterwards sent a message to the Admiralty to say "Mincemeat completed", and continued on to Gibraltar.

The body of "Major Martin" was found at around 9:30 am on 30 April 1943 by a local fisherman; it was taken to Huelva by Spanish soldiers, where it was handed over to a naval judge. Haselden, as vice-consul, was officially informed by the Spaniards; he reported back to the Admiralty that the body and briefcase had been found. A series of pre-scripted diplomatic cables were sent between Haselden and his superiors, which continued for several days. The British knew that these were being intercepted and, although they were encrypted, the Germans had broken the code; the messages played out the story that it was imperative that Haselden retrieve the briefcase because it was important.

At midday on 1 May an autopsy was undertaken on Michael's body; Haselden was present and – in order to minimise the possibilities that the two Spanish doctors would discover that the body was a three-month-old corpse – asked if, in the heat of the day and smell of the corpse, the doctors should bring the post mortem to a close and have lunch. They agreed and signed a death certificate for Major William Martin for "asphyxiation through immersion in the sea"; the body was released by the Spanish and, as Major Martin, was buried in the San Marco section of Nuestra Señora cemetery in Huelva, with full military honours on 2 May.

The Spanish navy retained the briefcase and, despite pressure from Adolf Clauss and some of his Abwehr agents, neither it nor its contents were handed over to the Germans. On 5 May the briefcase was passed to the naval headquarters at San Fernando near Cadiz, for forwarding to Madrid. While at San Fernando the contents were photographed by German sympathisers, but the letters were not opened. Once the briefcase arrived in Madrid, its contents became the focus of attention of Karl-Erich Kühlenthal, one of the most senior Abwehr agents in Spain. He asked Admiral Wilhelm Canaris, the head of the Abwehr, to personally intervene and persuade the Spanish to surrender the documents. Acceding to the request, the Spanish removed the still-damp paper by tightly winding it around a probe into a cylindrical shape, and then pulling it out between the envelope flap – which was still closed by a wax seal – and the envelope body. The letters were dried and photographed, then soaked in salt water for 24 hours before being re-inserted into their envelopes, without the eyelash that had been planted there. The information was passed to the Germans on 8 May. This was deemed so important by the Abwehr agents in Spain that Kühlenthal personally took the documents to Germany.

On 11 May the briefcase, complete with the documents, was returned to Haselden by the Spanish authorities; he forwarded it to London in the diplomatic bag. On receipt, the documents were forensically examined, and the absence of the eyelash noted. Further tests showed that the fibres in the paper had been damaged by folding more than once, which confirmed that the letters had been extracted and read. An additional test was made as the papers – still wet by the time they returned to London – were dried out: the folded paper dried into the rolled form it had when the Spaniards had extracted it from the envelope. To allay any potential German fears that their activities had been discovered, another pre-arranged encrypted but breakable cable was sent to Haselden stating that the envelopes had been examined and that they had not been opened; Haselden leaked the news to Spaniards known to be sympathetic to the Germans.

Final proof that the Germans had been passed the information from the letters came on 14 May when a German communication was decrypted by the Ultra source of signals intelligence produced by the Government Code and Cypher School (GC&CS) at Bletchley Park. The message, which had been sent two days previously, warned that the invasion was to be in the Balkans, with a feint to the Dodecanese. A message was sent by Brigadier Leslie Hollis – the secretary to the Chiefs of Staff Committee – to Churchill, then in the United States. It read "Mincemeat swallowed rod, line and sinker by the right people and from the best information they look like acting on it."

Montagu continued the deception to reinforce the existence of Major Martin, and included his details in the published list of British casualties which appeared in The Times on 4 June. By coincidence, also published that day were the names of two other officers who had died when their plane was lost at sea, and opposite the casualty listings was a report that the film star Leslie Howard had been shot down by the Luftwaffe and died in the Bay of Biscay; both stories gave credence to the Major Martin story.

On 14 May 1943 Grand Admiral Karl Dönitz met Hitler to discuss Dönitz's recent visit to Italy, his meeting with the Italian leader Benito Mussolini and the progress of the war. The Admiral, referring to the Mincemeat documents as the "Anglo-Saxon order", recorded

The Führer does not agree with ... [Mussolini] that the most likely invasion point is Sicily. Furthermore, he believes that the discovered Anglo-Saxon order confirms the assumption that the planned attacks will be directed mainly against Sardinia and the Peloponnesus.

Hitler informed Mussolini that Greece, Sardinia and Corsica must be defended "at all costs", and that German troops would be best placed to do the job. He ordered that the experienced 1st Panzer Division be transferred from France to Salonika, Greece. The order was intercepted by GC&CS on 21 May. By the end of June, German troop strength on Sardinia had been doubled to 10,000, with fighter aircraft also based there as support. German torpedo boats were moved from Sicily to the Greek islands in preparation. Seven German divisions transferred to Greece, raising the number present to eight, and ten were posted to the Balkans, raising the number present to eighteen.

On 9 July the Allies invaded Sicily in Operation Husky. German signals intercepted by GC&CS showed that even four hours after the invasion of Sicily began, twenty-one aircraft left Sicily to reinforce Sardinia. For a considerable time after the initial invasion, Hitler was still convinced that an attack on the Balkans was imminent, and in late July he sent General Erwin Rommel to Salonika to prepare the defence of the region. By the time the German high command realised the mistake, it was too late to make a difference.

On 25 July 1943, as the battle for Sicily went against the Axis forces, the Italian Grand Council of Fascism voted to limit the power of Mussolini, and handed control of the Italian armed forces over to King Victor Emmanuel III. The following day Mussolini met the King, who dismissed him as prime minister; the former dictator was then imprisoned. A new Italian government took power and began secret negotiations with the Allies. Sicily fell on 17 August after a force of 65,000 Germans held off 400,000 American and British troops long enough to allow many of the Germans to evacuate to the Italian mainland.

The military historian Jon Latimer observes that the relative ease with which the Allies captured Sicily was not entirely because of Mincemeat, or the wider deception of Operation Barclay. Latimer identifies other factors, including Hitler's distrust of the Italians, and his unwillingness to risk German troops alongside Italian troops who may have been on the point of a general surrender. The military historian Michael Howard, while describing Mincemeat as "perhaps the most successful single deception operation of the entire war", considered Mincemeat and Barclay to have less impact on the course of the Sicily campaign than Hitler's "congenital obsession with the Balkans". Macintyre writes that the exact impact of Mincemeat is impossible to calculate. Although the British had expected 10,000 killed or wounded in the first week of fighting, only a seventh of that number became casualties; the navy expected 300 ships would be sunk in the action, but they lost 12. The predicted 90-day campaign was over in 38.

Smyth writes that as a result of Husky, Hitler suspended the Kursk offensive on 13 July. This was partly because of the performance of the Soviet army, but partly because he still assumed that the Allied landing on Sicily was a feint that preceded the invasion in the Balkans, and he wanted to have troops available for fast deployment to meet them. Smyth observes that once Hitler gave up the initiative to the Soviets, he never regained it.

Montagu was appointed an Officer of the Order of the British Empire in 1944 for his part in Operation Mincemeat; for masterminding the plan, Cholmondeley was appointed a Member of the Order in 1948. Duff Cooper, a former cabinet minister who had been briefed on the operation in March 1943, published the spy novel Operation Heartbreak (1950), which contained the plot device of a corpse – with papers naming him as William Maryngton – being floated off the coast of Spain with false documents to deceive the Germans. The British security services decided that the best response was to publish the story of Mincemeat. Over the course of a weekend Montagu wrote The Man Who Never Was (1953), which sold two million copies and formed the basis for a 1956 film. The security services did not give Montagu complete freedom to reveal operational details, and he was careful not to mention the role played by signals intelligence to confirm that the operation had been successful. He was also careful to obscure "the idea of an organised programme of strategic deception ... with Mincemeat being presented as a 'wild' one-off caper". In 1977 Montagu published Beyond Top Secret U, his wartime autobiography which gave further details of Mincemeat, among other operations. In 2010 the journalist Ben Macintyre published Operation Mincemeat, a history of the events.

A 1956 episode of The Goon Show, titled "The Man Who Never Was", was set during the Second World War, and referred to a microfilm washed up on a beach inside a German boot. The play Operation Mincemeat, written by Adrian Jackson and Farhana Sheikh, was first staged by the Cardboard Citizens theatre company in 2001. The work focused on Michael's homelessness. In his book The Double Agents, the writer W. E. B. Griffin depicts Operation Mincemeat as an American operation run by the Office of Strategic Services. Fictional characters are blended with Ian Fleming and the actors David Niven and Peter Ustinov.

The story was the basis for the 2014 musical Dead in the Water, performed at the Camden, Brighton and Guildford Fringe Festivals in 2014. In 2015 the Welsh theatre company Theatr na nÓg produced Y dyn na fu erioed (The Man Who Never Was), a musical based on the operation and Glyndwr Michael's upbringing in Aberbargoed. The musical was performed by primary school children from Caerphilly County Borough during that year's Eisteddfod yr Urdd. Another musical, Operation Mincemeat, is based on the operation; it was initially staged in 2019 at the New Diorama Theatre, London, then moved to the Fortune Theatre in 2023. In 2014 a BBC television miniseries, Fleming: The Man Who Would Be Bond, dramatised some aspects of Operation Mincemeat, and Fleming's connection to the operation. In 2022 the film Operation Mincemeat was released, with Colin Firth as Montagu and Matthew Macfadyen as Cholmondeley.

The Commonwealth War Graves Commission took responsibility for Major Martin's grave in Huelva in 1977. In 1997 the Commission added the postscript "Glyndwr Michael served as Major William Martin RM". In November 2021 the Jewish American Society for Historic Preservation, working with the Association of Jewish Ex-Servicemen and Women and the London Borough of Hackney, placed a memorial at the Hackney Mortuary.

• Operation Copperhead – 1944 military deception operation
• Operation Bodyguard – World War II deception plan during the build-up to the 1944 Normandy landings

• BBC article on Operation Mincemeat