Dennis admitted to little aptitude for schooling and was expelled from Dulwich College.
His first novel published, The Forbidden Territory, was an immediate success when issued by Hutchinson in 1933, being reprinted seven times in seven weeks.
Wheatley mainly wrote adventure stories, with many books in a series of linked works. Background themes included the French Revolution (the Roger Brook series), Satanism (the Duke de Richleau series), World War II (the Gregory Sallust series) and espionage (the Julian Day series).
His writing is very descriptive and in many works, he manages to involve his characters with actual historical events while meeting real people. For example, in the Roger Brook series, the main character involves himself with Napoleon and Joséphine whilst being a spy for Prime Minister William Pitt the Younger. Similarly, in the Gregory Sallust series, Sallust shares an evening meal with Hermann Göring.
During the 1930s, he conceived a series of mysteries, presented as case files, with testimonies, letters, pieces of evidence such as hairs or pills. The reader had to read the evidence to solve the mystery before unsealing the last pages of the file, which gave the answer. Four of these 'Crime Dossiers' were published: Murder Off Miami, Who Killed Robert Prentice, The Malinsay Massacre, and Herewith The Clues.
In the 1960s, Hutchinson was selling a million copies of his books per year, and most of his titles were kept available in hardcover. A few of his books were made into films by Hammer, of which the best known is The Devil Rides Out (book 1934, film 1968). Wheatley also wrote non-fiction works, including an account of the Russian Revolution, a life of King Charles II of England, and several autobiographical volumes. He was considered an authority on the supernatural, satanism, the practice of exorcism, and black magic, to all of which he was hostile. During his study of the paranormal, though, he joined the Ghost Club.
Wheatley invented a number of board games including Invasion(1938), Blockade(1939) and Alibi (April 1953). He edited several collections of short stories, and from 1974 through 1977, he supervised a series of 45 paperback reprints for the British publisher Sphere with the heading "The Dennis Wheatley Library of the Occult", selecting the titles and writing short introductions for each book. These included both occult-themed novels by the likes of Bram Stoker and Aleister Crowley (with whom he once shared a lunch) and non-fiction works on magic, occultism, and divination by authors such as the Theosophist H. P. Blavatsky, the historian Maurice Magre, the magician Isaac Bonewits, and the palm-reader Cheiro.
Two weeks before his death in November 1977, Wheatley received conditional absolution from his old friend Cyril ‘Bobby’ Eastaugh, the Bishop of Peterborough. He was cremated at Tooting and his ashes interred at Brookwood Cemetery. He is commemorated on the Baker/Yeats family monument at West Norwood Cemetery.
His estate library was sold in a catalog sale by Basil Blackwell's in 1979. It suggested a well-read individual with wide-ranging interests particularly with respect to historical fiction and Europe.
His grandson Dominic Wheatley became one of the co-founders of the software house Domark, which published a number of titles in the 1980s and 1990s.