War hath no fury like a non-combatant.

He was educated at the City of London School and Balliol College Oxford. Nevinson would later write that "Montague is the only man I know whose white hair in a single night turned dark through courage. " He began as a grenadier-sergeant and rose to lieutenant and then captain of intelligence in 1915.

He finally retired in 1925 and settled down to become a full-time writer in the last years of his life. He died in 1928 at the age of 61. He returned to the Guardian but felt that his role was diminishing as the years passed.