Hector Hugh Munro was born in Akyab (now Sittwe), British Burma, which was then part of British India. Saki was the son of Charles Augustus Munro, an Inspector General for the Indian Imperial Police, and his wife, Mary Frances Mercer (1843–1872), the daughter of Rear Admiral Samuel Mercer. Her nephew Cecil William Mercer became a novelist under the name 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.
After his wife's death Charles Munro sent his children, including two-year-old Hector, home to England. The children were sent to Broadgate Villa, in Pilton near Barnstaple, North Devon, to be raised by their grandmother and paternal maiden aunts, Charlotte and Augusta, in a strict and puritanical household. It is said that his aunts were most likely models for some of his characters, notably the aunt in "The Lumber Room" and the guardian in "Sredni Vashtar": Munro's sister Ethel said that the aunt in "The Lumber Room" was an almost perfect portrait of Aunt Augusta. Munro and his siblings led slightly insular lives during their early years and were educated by governesses. At the age of 12 the young Hector Munro was educated at Pencarwick School in Exmouth and then as a boarder at Bedford School.
In 1887, after his retirement, his father returned from Burma and embarked upon a series of European travels with Hector and his siblings.
Hector followed his father in 1893 into the Indian Imperial Police and was posted to Burma, but successive bouts of fever caused his return home after only fifteen months.
In 1896, he decided to move to London to make a living as a writer.
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, but proved to be something of a false start.
While writing The Rise of the Russian Empire, he made his first foray into short story writing and published a piece called 'Dogged' in St Paul's in February 1899. He then moved into the world of political satire in 1900 with a collaboration with Francis Carruthers Gould entitled "Alice in Westminster". Gould produced the sketches, and Munro wrote the text accompanying them, using the pen-name "Saki" for the first time. The series lampooned political figures of the day ('Alice in Downing Street' begins with the memorable line, '"Have you ever seen an Ineptitude?"' - referring to a zoomorphised Arthur Balfour), and was published in the Liberal Westminster Gazette.
In 1902 he moved to The Morning Post, described as one of the 'organs of intransigence' by Stephen Koss, to work as a foreign correspondent, first in the Balkans, and then in Russia, where he was witness to the 1905 revolution in St Petersburg. He then went on to Paris, before returning to London in 1908, where 'the agreeable life of a man of letters with a brilliant reputation awaited him.' In the intervening period Reginald had been published in 1904, the stories having first appeared in The Westminster Gazette, and all this time he was writing sketches for The Morning Post, the Bystander, and The Westminster Gazette. He kept a place in Mortimer Street, wrote, played bridge at the Cocoa Tree Club, and lived simply. Reginald in Russia appeared in 1910, and The Chronicles of Clovis was published in 1911, and Beasts and Super-Beasts in 1914, along with many other short stories that appeared in newspapers not published in collections in his lifetime.
He also produced two novels, The Unbearable Bassington (1912) and When William Came (1913).
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 was promoted to lance sergeant. More than once he returned to the battlefield when officially 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, which appeared at the beginning of The Square Egg and Other Sketches (1924). Rothay Reynolds, a close friend, wrote a relatively lengthy memoir in The Toys of Peace (1919), but aside from this, the only other biographies of Munro are Saki: A Life of Hector Hugh Munro (1982) by A. J. Langguth, and The Unbearable Saki (2007) by Sandie Byrne. All later biographies have had to draw heavily upon Ethel's account of her brother's life.
Munro was homosexual at a time when in Britain 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".