Hector Hugh Munro was born in Akyab, British Burma, which was then still part of the British Raj, and was governed from Calcutta under the authority of the Viceroy of India. Saki was the son of Charles Augustus Munro, an Inspector General for the Indian Imperial Police, By his marriage to Mary Frances Mercer (1843–1872), the daughter of Rear Admiral Samuel Mercer. Her nephew, Cecil William Mercer, later became a famous novelist as Dornford Yates.
In 1872, on a home visit to England, Mary Munro was charged by a cow, and the shock caused her to miscarry. She never recovered and soon died. Charles Munro sent his children, including two-year-old Hector, home to England, where they were brought up by their grandmother and aunts in a strict and puritanical household.
The young Hector Munro was educated at Pencarwick School in Exmouth and then as a boarder at Bedford School. On a few occasions after he retired from Burma, Charles Munro travelled with Hector and his sister to fashionable European spas and resorts.
In 1893 Hector Munro followed his father into the Indian Imperial Police and was posted to Burma. Two years later, having contracted malaria, he resigned and returned to England.
At the start of the First World War Munro was 43 and officially over-age to enlist, but he refused a commission and joined the 2nd King Edward's Horse as an ordinary trooper. He later transferred to the 22nd Battalion of the Royal Fusiliers, in which he rose to the rank of lance sergeant. More than once he returned to the battlefield when officially still too sick or injured. In November 1916 he was sheltering in a shell crater near Beaumont-Hamel, France, during the Battle of the Ancre, when he was killed by a German sniper. According to several sources, his last words were "Put that bloody cigarette out!"
Munro has no known grave. He is commemorated on Pier and Face 8C 9A and 16A of the Thiepval Memorial.
In 2003 English Heritage marked Munro's flat at 97 Mortimer Street, in Fitzrovia with a blue plaque.
After his death his sister Ethel destroyed most of his papers and wrote her own account of their childhood.
Munro was homosexual, but in Britain at that time sexual activity between men was a crime. The Cleveland Street scandal (1889), followed by the downfall of Oscar Wilde (1895), meant "that side of [Munro's] life had to be secret".
Munro was a Tory and somewhat reactionary in his views.
The pen name "Saki" may be a reference to the cupbearer in the Rubáiyát of Omar Khayyam, a poem mentioned disparagingly by the eponymous character in "Reginald on Christmas Presents" and alluded to in a few other stories. This reference is stated as fact by Emlyn Williams in his introduction to a Saki anthology published in 1978. However, "Saki" may also or instead be a reference to the South American monkey of that name, which at least two commentators, Tom Sharpe and Will Self, have connected to the "small, long-tailed monkey from the Western Hemisphere" that is a central character in "The Remoulding of Groby Lington".
Munro started his writing career as a journalist for newspapers such as the Westminster Gazette, the Daily Express, the Morning Post, and magazines such as the Bystander and Outlook. His first book The Rise of the Russian Empire, a historical study modelled upon Edward Gibbon's The Decline and Fall of the Roman Empire, appeared in 1900, under his real name.
From 1902 to 1908 Munro worked as a foreign correspondent for the Morning Post in the Balkans, Warsaw, Russia (where he witnessed Bloody Sunday), and Paris. He then gave up foreign reporting and settled in London. Many of his stories from this period feature Reginald and Clovis, young men-about-town who take mischievous delight in the discomfort or downfall of their conventional, pretentious elders.
Shortly before the First World War, when "invasion literature" was selling well, Munro published a "what-if" novel, When William Came, subtitled "A Story of London Under the Hohenzollerns", imagining the eponymous German emperor conquering and occupying Britain.