It was entered into the 1956 Cannes Film Festival. Nigel Balchin's screenplay won the BAFTA for that year.
In 1943, Royal Navy Lieutenant Commander Ewen Montagu (Clifton Webb) comes up with a scheme to deceive the Nazis about the impending invasion of southern Europe. It entails releasing a dead body just off the coast of Spain, where strong currents will almost certainly cause it to drift ashore in an area where a skilled German secret agent operates. The corpse will appear as a plane crash victim, the non-existent Royal Marine Major William Martin, who is carrying false letters about a forthcoming Allied invasion of Greece, rather than the obvious target of Sicily. Time is short, but the impatient Montagu is finally given approval to carry out the mission.
On the advice of a medical expert, Montagu procures the body of a man who died of pneumonia (so that he will seem to have drowned) from the grieving father. Then, after proper preparations, he and his assistant, Lt. Acres (Robert Flemyng), take the corpse (concealed in a canister) to a waiting submarine. The submarine travels to the Mediterranean, along the way evading a depth-charge attack, before surfacing at night to release the body. As hoped, the body washes ashore on a Spanish beach and is processed by local authorities, observed by German and British consulate staff. After the attache case containing the letters is returned to London, as part of the personal possessions of the deceased Major Martin, Montagu is disappointed the documents do not appear to have been tampered with. However, a laboratory expert assures him the key letter describing the (false) Allied attack in Greece was cleverly opened and resealed.
Hitler is convinced the document is genuine even though a German intelligence officer is skeptical. He orders an IRA Nazi spy Patrick O'Reilly (Stephen Boyd) dispatched to London to investigate. What O'Reilly uncovers is inconclusive, in his mind, until he checks out Martin's "fiancée", Lucy Sherwood (Gloria Grahame). She is the roommate of Montagu's assistant, Pam (Josephine Griffin). O'Reilly shows up at their flat, posing as Martin's old friend, on the same day Lucy received news that her real pilot boyfriend has been killed in action. Her genuine grief mostly convinces O'Reilly. As a final test, however, he leaves the address of his lodgings in north London, telling Lucy to contact him if she needs anything. He then informs his German superiors by radio to expect a message from him in an hour, unless British counterintelligence comes for him. Montagu almost makes a mistake, but realises in time why O'Reilly left his address and, with some difficulty, convinces his superior to order O'Reilly is not arrested. O'Reilly then sends a "Martin genuine!" radio message, and the Nazi's transfer some of their Sicily-based forces to Greece, making the Allied deceit successful.
After the war, Montagu leaves a medal he was awarded at the grave of the man who never was.
Operation Mincemeat involved the acquisition and dressing up of a human cadaver as a "Major William Martin, R.M." and putting it into the sea near Huelva, Spain. Attached to the dead body was a brief-case containing fake letters falsely stating that the Allied attack would be against Sardinia and Greece rather than Sicily, the actual point of invasion. When the body was found, with all the false pocket litter data, the Spanish Intelligence Service passed copies of the papers to the German Intelligence Service which passed them on to their High Command. The ruse was so successful that the Germans still believed that Sardinia and Greece were the intended objectives, weeks after the landings in Sicily had begun.
The screenplay of the film stayed as close to the truth as was convenient, with the remainder being fiction. For example, the Irish spy in the film is a complete fabrication; The British Secret Service controlled the German spy network in the UK with its Double-Cross System, though this fact was still classified at the time the film was made. Ewen Montagu declared that he was happy with the fictitious incidents which, although they did not happen, might have happened. During filming, Montagu has a cameo role, that of a Royal Air Force air vice-marshal who has doubts about the feasibility of the proposed plan. It was described as a "surreal" moment when the real Montagu addressed his fictional persona, played by Webb.
The film earned an estimated $1.1 million in North American rentals in 1956.
The BBC's popular radio comedy show, The Goon Show, made a send-up of the story of The Man Who Never Was and incorporated most of the regular Goon Show characters. Written by Spike Milligan and Larry Stephens, it was broadcast on 20 March 1956 and remade later and broadcast on 17 February 1958. Coincidentally, Peter Sellers (one of the Goons) provided the voice of Winston Churchill in the film, although the character did not appear in The Goon Show adaptation.