After the war he completed his education in Scotland, graduating with a Bachelor of Commerce from the University of Edinburgh in 1925 before becoming an auditor, and moved to France where he worked in the Paris branch of Peat Marwick Mitchell.

Returning to England he rejoined the military, initially serving in the Royal Army Pay Corps as a lieutenant.

He was promoted to captain in Intelligence, assisting the French underground, and then was a lieutenant-colonel in the Displaced Persons Division in Austria.

A Roman Catholic convert,[5] Marshall wrote stories that are usually humorous and mildly satiric and typically have religious overtones.

Marshall's first literary work was a collection of short stories entitled A Thief in the Night published while he was still a student at St. Andrews University.

A stream of novels soon followed, but none of the fiction he wrote before the Second World War gained as much notoriety or staying power as Father Malachy's Miracle (1931).

A number of his later novels also deal with clergy who are faced with temptation but manage to triumph in a modest and humble manner (e.g., The World, the Flesh, and Father Smith (AKA All Glorious Within) (1944), A Thread of Scarlet (AKA Satan and Cardinal Campbell) (1959), Father Hilary's Holiday (1965), The Month of the Falling Leaves (1963)).

Other books centered on religious issues after the Second Vatican Council, dealt increasingly with the internal conflict between Traditionalist Catholicism and Modernism in the Catholic Church, such as The Bishop (1970), Peter the Second (1976), Urban the Ninth (1973) and Marx the First (1975).

In 1961, the novel was the basis for the German film Das Wunder des Malachias directed by Bernhard Wicki and starring Horst Bollmann, Richard Münch and Christiane Nielsen.

His 1947 novel Vespers in Vienna, a highly critical depiction of the British military role in Operation Keelhaul, was the basis of the 1949 film The Red Danube starring Walter Pidgeon, Ethel Barrymore, Peter Lawford, Angela Lansbury and Janet Leigh.

His 1953 novel The Fair Bride was the basis of the 1960 film The Angel Wore Red starring Ava Gardner, Dirk Bogarde, Joseph Cotten and Vittorio De Sica.

His 1952 book, The White Rabbit, recounting the World War II exploits of Special Operations Executive agent F. F. E. Yeo-Thomas, was made into a TV mini-series in 1967.