Born at his grandfather's house in Surrey, England, Dennis was the son of Lt.-Col. Michael Frederic Beauchamp Dennis of the King's Own Scottish Borderers, who came of an old Devonshire family, and Louise, née Bosanquet, whose ancestors were bankers of Huguenot origin. The family moved to Southern Rhodesia (now Zimbabwe) and after his father's death in action in 1918, his mother married Fitzroy Griffin. Dennis attended school in Rhodesia. At fifteen, he joined his uncle, Ernan Forbes Dennis, a British diplomat working in Vienna as Passport Control Officer (a cover for his real role as MI6 Head of Station with responsibility for Austria, Hungary and Yugoslavia), and his wife, Phyllis Bottome, the novelist. He would travel to Germany for further education before returning to the UK, where he stayed for four years before settling in the U.S. in 1934.

Dennis held jobs at the National Board of Review of Motion Pictures, a censorship body; The New Republic, a progressive political journal; and Time. His job at Time returned him to Britain in 1950 (or 1949). Easing into novel writing, in 1949 he published his first acknowledged novel, Boys and Girls Come out to Play (A Sea Change in the USA), which won the Anglo-American novel award for that year (shared with Anthony West). It starts semi-autobiographically, with a depiction of a young man having an epileptic fit, a condition Dennis suffered from all his life. Later in 1955, Dennis published his most notable work, Cards of Identity, a witty psychological satire that gained cult acclaim. The novel was converted into a play the next year. Dennis's career would involve a mixture of non-fiction, novel, criticism, and play writing. Starting in 1961, his book reviews would appear in the Sunday Telegraph for two decades. He began as a contributor for Encounter, a cultural-literary magazine, in 1963, and would eventually become a co-editor before terminating his relationship in 1970.

Dennis's books were few but distinguished; his other works include Two Plays and a Preface (1958), Dramatic Essays (1962), the novel A House in Order (1966), a short study of Jonathan Swift, which won the Royal Society of Literature award under the W. H. Heinemann bequest in 1966, Exotics: Poems of the Mediterranean and Middle East (1970), and An Essay on Malta (1972).

Three of his plays were put on at the Royal Court theatre: Cards of Identity (1956), The Making of Moo (1957) and August for the People (1962). The first London revival of The Making of Moo was staged at the Orange Tree Theatre in November 2009 [1].

According to a letter published in The Guardian in May 2008: "In the 1930s, Dennis wrote Chalk and Cheese under the pseudonym Richard Vaughan. Legend has it that, before publication, every copy was destroyed in an air raid on a warehouse."

Dennis was married twice: (1) Marie-Madeleine Massias, from Charente- Maritime, France. Two daughters, Frederica Freer and Michie Herbert, sculptor. (2) Beatrice Ann Hewart Matthew, actress.

Dennis spent his last years in Malta. He died in Gloucestershire, July 1989.