The family moved to Southern Rhodesia (now Zimbabwe) and after his father's death in action in 1918, his mother married Fitzroy Spencer Griffin.

At fifteen he joined his uncle, Ernan Forbes Dennis (a British diplomat who had worked as Vice-Consul in Vienna as a cover for his real role as MI6 Head of Station with responsibility for Austria, Hungary and Yugoslavia), and his wife, the novelist Phyllis Bottome, at their school in Kitzbühel.

Dennis held jobs at the National Board of Review of Motion Pictures, a censorship body; The New Republic, a progressive political journal; and Time.

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).

Later in 1955, Dennis published his most notable work, Cards of Identity, a witty psychological satire that gained cult acclaim.

A short study of Jonathan Swift won the Royal Society of Literature Award under the W. H. Heinemann bequest in 1966; this was followed by Exotics: Poems of the Mediterranean and Middle East (1970) and his last book, An Essay on Malta (1972), with illustrations by Osbert Lancaster.

According to a letter published in The Guardian in May 2008: "In the 1930s, Dennis wrote Chalk and Cheese; a co-educational school novel under the pseudonym Richard Vaughan.