After beginning his writing career as a journalist he wrote more than 50 books, some of which were made into films, including his 1978 historical novel Boarding Party, the story of a secret incident from the Second World War, which became The Sea Wolves (1980) starring Gregory Peck, Roger Moore and David Niven.

In the Far East, whilst sailing in convoy HC-44 from Calcutta to Chittagong, his troopship, the El Madina, was torpedoed on 16 March 1944[7] Ten crew, six gunners and 364 troops died and Leasor spent 18 hours adrift in the Indian Ocean.

During the war Leasor wrote over 300 news stories on the Burma campaign, and also contributed features for the BBC and All India Radio as well as for virtually every British national newspaper.

After the war Leasor went to Oriel College, Oxford, where he read English, edited The Isis and wrote articles for The Wide World magazine.

[1][4] He was first noticed as an author after writing a number of critically acclaimed histories, including The Red Fort (1956), his account of the Indian Mutiny, about which Cecil Woodham-Smith commented in the New York Times: "Never has this story of hate, violence, courage and cowardice been better told";[13] The One That Got Away (1956), the story of the only German prisoner of war, Franz von Werra, to escape from Allied territory during World War 2, which was later made into a film starring Hardy Kruger; The Plague and the Fire (1961), about London's twin disasters in the 17th century; The Millionth Chance (1957) about the loss of the R101 airship; and Singapore: The Battle that Changed the World (1968), on the Battle of Singapore in 1942.

Leasor became a full-time author in the 1960s after the success of his novel Passport to Oblivion (1964), a thriller featuring Dr Jason Love, which became one of the best selling books of the decade and was filmed as Where the Spies Are (1965) starring David Niven.

He continued to write historical books: later titles included Green Beach (1975) about the Dieppe Raid of 1942 and a secret operation to capture German radar equipment; Boarding Party (1978), based on the real events of Operation Creek in 1943, which was filmed as The Sea Wolves (1980); The Unknown Warrior (1980) about an agent who was a major part of the D-Day deception plans; and Who Killed Sir Harry Oakes?

(1983), which was made into a TV mini-series in 1989 called Passion and Paradise, starring Armand Assante, Catherine Mary Stewart, Mariette Hartley and Kevin McCarthy, with Rod Steiger playing Sir Harry Oakes.

Leasor wrote six books under the pseudonym Andrew MacAllan,[16] and ghosted 'autobiographies' for people as diverse as King Zog of Albania—who made him a member of the Order of St John of Jerusalem[17]—and British actors Kenneth More and Jack Hawkins.

[18] Leasor had a great interest in cars and owned pre-war specimens such as a rare Cord roadster, which he made use of in the Jason Love novels,[19][20] and an SS Jaguar 100 which featured in his Aristo Autos series.

Leasor wrote the first three chapters and a synopsis of an epic historical saga, loosely based on the origins of Hong Kong-based trading companies like Jardine Matheson and Swire, and Gillon Aitken took it with the agreed story about MacAllan around London-based publishers.

With the co-operation of a bank manager, he opened an account in London in the name of Andrew MacAllan, so that any royalty payments could be made without arousing suspicion.