Marshall was born in Edinburgh, Scotland, the son of Claude Niven Marshall and Annie Margaret (Bruce) Marshall. He was educated at St. Andrews. He became a Roman Catholic in 1917 and remained active and interested in the faith for the rest of his life. He was a member and at times served as an officer in the Una Voce and the Latin Mass Society organizations.

During World War I he initially served as a private in the Highland Light Infantry. He was commissioned as a second lieutenant into the Royal Irish Fusiliers in 1918. Six days before the 1918 Armistice he was seriously wounded. Courageous German medical orderlies risked intense shelling to rescue him and he was taken prisoner. His injuries resulted in the amputation of one leg. He was promoted to lieutenant in 1919 and invalided out in 1920.

After the war he completed his education in Scotland, became an auditor, and moved to France where he worked in the Paris branch of Peat Marwick Mitchell.

In 1928 he married Mary Pearson Clark (1908 – 1987). They had one daughter — Sheila Elizabeth Bruce Marshall. In 2009, his granddaughter, Leslie Ferrar, was Treasurer to the Prince of Wales.

He was living in Paris during the 1940 Invasion of France and escaped two days before the Nazi occupied the city. 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. He transferred to the General List in 1945, and left the Army as a lieutenant - colonel in 1946.

After the war Marshall returned to France, moving to the Côte d'Azur and living there for the remainder of his life. He died in Biot, France, six days before his 88th birthday.

A Roman Catholic convert, his stories are usually humorous and mildly satiric and typically have religious overtones. Important themes which run through his works are Catholicism, accounting, a Scottish heritage and war, adventure and intrigue. Often major characters are accountants or Catholic priests. Characters in his novels are often fond of animals and concerned about their treatment. Contempt for modern art and literature is often expressed.

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. His first novel, This Sorry Scheme was published in 1924. 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).

After the Second World War Marshall became a writer full - time, giving up his work as an accountant. As to his dual career as an accountant and writer, Marshall once said, "I am an accountant who writes books. In accounting circles I am hailed as a great writer. Among novelists I am assumed to be a competent accountant."

Among his better known works after the Second World War is The White Rabbit (1953), a biography of Wing Commander F. F.E. Yeo - Thomas, describing his exploits and sufferings while in the Resistance during World War II.

In 1959 he was awarded the Włodzimierz Pietrzak prize.

The theme of much of Marshall's works is religion, with a focus on Roman Catholicism. His first great success, Father Malachy's Miracle, is about an innocent Scottish priest whose encounter with sinful behavior causes him to become involved in a miracle. 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 deal more with Catholic doctrine and its relationship to modern life than with personal responsibility, such as The Bishop (1970), Peter the Second (1976), Urban the Ninth (1973) and Marx the First (1975).

Like many expatriates, Marshall expressed great love for his homeland. Most of his books were either set in Great Britain and/or have main characters of British nationality. The work which best shows Marshall's affection for Scotland may be The Black Oxen (1972), which Marshall billed as a Scottish Epic.

Several of Marshall's books have themes about espionage and intrigue, such as Luckypenny (1937), A Girl from Lübeck (1962), The Month of the Falling Leaves (1963), Operation Iscariot (1974), An Account of Capers (1988), The Accounting (AKA The Bank Audit) (1958), and Only Fade Away (1954).

Some of his novels feature major characters who, like Marshall himself, have suffered the loss of a limb. Often major characters from one novel appear in minor roles in other novels.

Marshall was relatively popular in his time. His books were reviewed in major publications on both sides of the Atlantic. At least two of his books were Book of the Month Club selections; Vespers in Vienna (1947) and The World, the Flesh, and Father Smith (AKA All Glorious Within) (1944), in June 1945. An Armed Services Edition of The World, the Flesh, and Father Smith was also produced.

His books were published in at least nine languages — English, Dutch, French, German, Italian, Polish, Czech, Portuguese & Spanish.

His 1931 novel Father Malachy's Miracle was adapted for the stage in 1938 by Brian Doherty. The novel was adapted for presentation on The Ford Theatre Hour, an American TV show, in 1950. 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 was the basis of the 1949 film The Red Danube starring Walter Pidgeon, Ethel Barrymore, Peter Lawford, Angela Lansbury and Janet Leigh. George Sidney directed. After the movie's release the novel was re-issued under the title The Red Danube. 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. It was the last film directed by Nunnally Johnson. His 1952 book, The White Rabbit, recounting the World War II exploits of secret agent F.F.E. Yeo - Thomas, was made into a TV mini - series in 1967. His 1963 novel The Month of the Falling Leaves was the basis of the 1968 German TV show Der Monat der Fallenden Blätter. Marshall co-wrote the screenplay with Herbert Asmodi. It was directed by Dietrich Haugk.