John Hay Beith was born in Chorlton, Lancashire, the second son of John Alexander Beith, of the Beith, Stevenson & Co. shipping and export business, based in Manchester. His paternal grandfather was a Presbyterian clergyman, Rev. Dr. Alexander Beith, active in the Disruption of 1843.

Beith was educated at Fettes College, Edinburgh (initially as fag to Sir John Simon) and at St. Johns College, Cambridge, where he played rugby and captained the college boat club. Finding a second-class degree in Classics insufficient to obtain a steady teaching position at a major school, he returned to Cambridge for a further year to qualify as a teacher of physics and chemistry.

Beith joined Durham School in 1902 as junior science master, and also coached the rugby and boating crews. Durham featured in his The Housemaster (1936).

Hay later taught at Fettes, spending much of his leisure time in writing. His first novel, the schoolboy romance Pip (1907), appeared while he was still teaching at the school. His light novels combined humour and shrewd observation, with an English tolerance of eccentricity and suited the taste of the age (he collaborated with P.G.Wodehouse)

He was a second-lieutenant in the Argyll and Sutherland Highlanders and was in France in April 1915 and was one of the first 100,000 of Kitchener's Army. He was awarded the Military Cross. He was later Director of Public Relations at the War Office (1938-1941).

His work was well known for its wit; often quoted is this line from his play, Housemaster: "What do you mean, funny? Funny-peculiar or funny ha-ha?" From the same play, two characteristic Hay lines, from masters' reports on their pupils:

• ‘He can translate English into a Greek not spoken in Greece, and Greek into an English not spoken anywhere, with equal facility’
• ‘Despite his natural levity he habitually gravitates towards the bottom.’

The First Hundred Thousand (1916) is his best-known work, and is marked by the same sharp sense of humor as his other work: "War is hell, and all that, but it has a good deal to recommend it. It wipes out all the small nuisances of peace-time."

All In It K(1) Carries On: A Continuation of the First Hundred Thousand (1917) and Carrying On (1917) were also popular books of his. Other works include Tilly of Bloomsbury 1919, The Right Stuff, The Crimson Cocoanut, A Man's Man, A Safety Match, and Happy-Go-Lucky.

In 1928 Beith adapted P. G. Wodehouse's novel A Damsel in Distress as a play. In 1929 Wodehouse helped to adapt Beith's Story Baa Baa Black Sheep for the stage and in 1930 they again collaborated on the dramatisation of Wodehouse's Leave it to Psmith.

Beith served as Technical Advisor for Cecil B. DeMille's silent extravaganza, "The Little American" (1917), starring Mary Pickford, and was responsible for screenplays/dialogue of fifteen films, including Alfred Hitchcock's The Thirty-Nine Steps and Secret Agent.

He died at Petersfield, Hampshire, on 22 September 1952.