Siegfried was born at Weirleigh, Kent, England, in 1886, the second son of Alfred and Theresa (née Thornycroft), who subsequently separated when Sassoon was five years old. Sassoon was educated at Marlborough and then at Clare College, Cambridge. He studied both Law and History at Cambridge before leaving without taking a degree. After leaving Cambridge, Sassoon lived the life of a sportsman, hunting, riding point-to-point races and playing cricket until the outbreak of the War.
Sassoon enlisted on 2 August 1914, two days before the British declaration of war, and initially joined as a trooper in the Sussex Yeomanry. However, after a riding accident whilst doing some field-work (he had put his horse at a fence blind with summer vegetation and a hidden strand of wire brought the horse down on top of him, leaving Sassoon with a badly broken right arm), Sassoon was commissioned in the Royal Welch Fusiliers (May 1915). Between November 1915 and April 1917 he served as a second lieutenant in both the First and Second Battalions R.W.F.
On November 1, 1915 Sassoon suffered his first personal loss of the War. His younger brother Hamo was buried at sea after being mortally wounded at Gallipoli. Sassoon subsequently commemorated this with a poem entitled "To My Brother" (published in the Saturday Review, February 26, 1916). Then on March 18, 1916 second lieutenant David C. Tommy' Thomas (the 'Dick Tiltwood' of Memoirs of a Fox-hunting Man) was killed whilst out with a wiring party. He had been hit in the throat by a rifle bullet, and despite the Battalion doctor being a throat specialist, had died of the wound.
These losses upset Sassoon and he became determined to "get his revenge" on the Germans. To this end, he went out on patrol in no-man's-land even when there were no raids planned. Such reckless enthusiasm earned him the nickname "Mad Jack", but he was saved from further folly by a four-week spell at the Army School in Flixecourt. Returning to the front a month later some of Sassoon's desire for revenge had abated, and when his platoon was involved in a raid on Kiel Trench shortly afterwards, his actions in getting all his men back to the British trenches earned him a Military Cross, which he received the day before the start of the Battle of the Somme, in July 1916. During the first day of the Battle of the Somme Sassoon was in reserve, in a support trench opposite Fricourt. He was not involved in the Battle of the Somme and was sent home from France in late July after an attack of trench fever (or enteritis). From Oxford's Somerville College, Sassoon was sent home to Weirleigh for convalescence. He reported to the Regimental Depot in Liverpool in December 1916, and returned to France in February 1917.
Sassoon was only back in France for two days before going down with German measles, which forced him to spend nearly ten days at the 25th Stationary Hospital in Rouen. On March 11 Sassoon rejoined the 2nd Battalion Royal Welch Fusiliers on the Somme front. He was "in reserve" during the Battle of Arras before spending a month in the Hindenburgh Tunnel. Sassoon participated in the Second Battle of the Scarpe where he was wounded in the shoulder. This particular incident started a train of events which culminated in Sassoon's "Declaration", for it was whilst on convalescent leave after being wounded that Sassoon talked to several prominent pacifists (including Middleton Murry and Bertrand Russell). His Declaration of "wilful defiance" was written during this time, and he returned to the Depot in Liverpool having sent his statement to his Colonel, miserably determined to take whatever punishment was meted out. Fortunately for Sassoon, his friend and fellow Welch Fusilier Robert Graves, intervened with the authorities and managed to persuade them to have Sassoon medically boarded (or referred), with the result that in July 1917 he was sent to Craiglockhart War Hospital, Edinburgh officially suffering from shell-shock.
It was at Craiglockhart that Sassoon met the poet Wilfred Owen, also diagnosed with shell-shock). Sassoon's encouragement of Owen's writing has been well-documented. Sassoon himself wrote a good deal of poetry whilst at Craiglockhart and the material he wrote at that time later appeared in Counter-Attack and Other Poems. After four months at Craiglockhart, Sassoon was again passed fit for General Service abroad. He had spent many hours talking to his psychiatrist, Dr. WHR Rivers and eventually realised that his protest had achieved nothing, except to keep him away from his men; his decision to apply for General Service seems to have been based on his perceived responsibilities at the front.
In November 1917 he was passed fit for General Service and returned to the Regimental Depot, from whence in January 1918, he was posted to Limerick. In February 1918, Sassoon was posted to Palestine with the 25th Battalion of the Royal Welch Fusiliers. After three months in Palestine the Battalion was posted to France and Sassoon eventually found himself in the Front Line near Mercatel. From there he moved to St. Hilaire and the Front Line at St. Floris where his old foolhardiness took over, despite the responsibility of being a Company Commander. Sassoon decided to attack the German trenches opposite them, and he went out with a young Corporal. His actions were paid for with a wound to his head on July 13, 1918, and Sassoon was invalided back to England. That was the end of Sassoon's War. After a period of convalescence he was placed on indefinite sick leave until after the Armistice, eventually retiring officially from the Army in March 1919.
Sassoon continued to write poetry after the war. He married Hester Gatty in 1933, and their son George was born in 1936. The marriage ended a few years later. He did not serve during the Second World War, but lived quietly at Heytesbury House in Wiltshire, where he died in 1967, one week short of his eighty-first birthday.