(1866–1946) married (1892) Elsie Clara Hopkins (d.1946). Born in London, the son of a leather merchant, he was educated at Wyggeston Grammar School, Leicester, but left aged 16 because of a family financial crisis. His first novel, Expiation (1887), was published with his father's money; his second did not appear for another seven years. He continued to work in the family firm, often travelling on the Continent and in America, while publishing stories in the Sheffield Weekly Telegraph and becoming an increasingly popular author of thrillers. Some have described Oppenheim's Mysterious Mr Sabin (1898) as the first spy novel. Mr Sabin is the prototype of the millionaire would-be Mr Fixits of twentieth-century crime fiction: he wants to restore the French monarchy with the help of his scientific inventions, which he will give to the Germans so that they can conquer the British. At about the age of 40 Oppenheim sold the firm and took to writing full-time. Much of his vast output (more than 160 volumes of fiction) was dictated to a secretary. Like Le Queux, Childers, and others he wrote fiction warning of the danger from Germany: A Maker of History (1905) turns on a plot for a joint Russian-German invasion. See also The Secret (1907). Like many of his fellow-authors he spent the First World War in the Ministry of Information. In 1922 he left England to live on the Riviera (Mecca to many of his characters), perhaps influenced by the prospect of paying tax on the profits of his most successful novel, The Great Impersonation (1920). There he lived ostentatiously, even for that frivolous world, and conducted a very public series of love affairs. In the 1920s many of his novels were first published in America in serial form, an arrangement immensely lucrative for Oppenheim: Collier's, for example, paid 20,000 dollars to publish The Treasure House of Martin Hews (1929). In 1935 he moved his domicile to Guernsey to avoid the French tax authorities, but then bought another house in France, to which he foolishly returned after the outbreak of war; he and his wife managed to escape to Portugal and arrived in England in 1941. In 1945 they were refused permission to return to Guernsey. Oppenheim, probably in an attempt to evade English death duties, chartered a small yacht to take him and his almost senile wife to their house in Guernsey, until recently the local headquarters of the Luftwaffe, where they both soon died.

Oppenheim published many orthodox detective stories, such as, Slane's Long Shots (1930), some futuristic novels, such as The Wrath to come (1924), and four plays, but his most distinctive and characteristic works are thrillers about international diplomacy and the secret service in which the hero or heroine works to save civilization, whether from Germans, Bolsheviks, or mad megalomaniacs. Like John Buchan, he was excited by the thought that the destinies of nations are shaped by a handful of powerful men, some wicked, some benevolent; like many of his imitators and followers, he let the reader identify with the ordinary man caught up in epoch-making events. Not for him, however, the rather later emphasis on the dull and squalid side of intelligence work: his novels take place in casinos, ballrooms, ocean liners, grand hotels, wagons-lits, and embassies; his characters are American millionaires, Russian princes, English aristocrats, and his heroines are invariably fascinating and seductive. Never again was international diplomacy to seem so glamorous and so romantic. Between 1908 and 1912 he published five novels as ‘Anthony Partridge’ which subsequently appeared under his own name. There is a biography by Ronald Standish (1957) and an autobiography (1941).