He studied at Dulwich College and graduated from Trinity College, Oxford in 1888. He was a contemporary of fellow Liberal Anthony Hope, who went on to write the adventure novel The Prisoner of Zenda.

His first novel, A Romance of Wastdale, was published in 1895. He was the author of more than 20 books, including At The Villa Rose (1910), a mystery novel in which he introduced his French detective, Inspector Hanaud. His best-known book is The Four Feathers, which has been made into several films. Many consider it his masterpiece. Other books are The House of the Arrow (1924), No Other Tiger (1927), The Prisoner in the Opal (1929) and Fire Over England (1937). He contributed a short story, The Conjurer, to The Queen's Book of the Red Cross.

Mason was elected as a Liberal Member of Parliament for Coventry in the 1906 general election. He served only a single term in Parliament, retiring at the next general election in January 1910.

Mason served with the Manchester Regiment in the First World War, being promoted Captain in December 1914. He transferred to the General List in 1915 and the Royal Marine Light Infantry in 1917 with the rank of Major. His military career included work in naval intelligence, serving in Spain and Mexico, where he set up counter-espionage networks on behalf of the British government.

He died in 1948 while working on a non-fiction book about Admiral Robert Blake. Mason had been offered a knighthood but declined it declaring that such honors meant little to a childless man.

Novels featuring Inspector Hanaud

• At the Villa Rose (1910)
• The House of the Arrow (1924)
• The Prisoner in the Opal (1928)
• They Wouldn't Be Chessmen (1934)
• The House in Lordship Lane (1946)

Other novels

• The Courtship of Maurice Buckler (1896)
• Lawrence Clavering (1897)
• The Philanderers (1897)
• Parson Kelly (1899) (with Andrew Lang)
• Clementina (1901)
• The Four Feathers (1902)
• The Truants (1904)
• Running Water (1906)
• The Broken Road (1907)
• A Romance of Wastdale (1910)
• Miranda of the Balcony (1911)
• The Turnstile (1912)
• The Witness for the Defence (1913)
• The Summons (1920)
• The Watchers (1924)
• The Winding Stair (1924)
• No Other Tiger (1927)
• The Dean's Elbow (1930)
• The Three Gentlemen (1932)
• The Sapphire (1933)
• Fire Over England (1936)
• The Drum (1937)
• Konigsmark (1938)
• Musk and Amber (1942)
• The Crystal Trench (2001)