Born into a wealthy banking family in 1886, Sassoon was educated at Marlborough College before going up to Cambridge, where he studied first Law, then History at Clare College. He did not complete his degree and left University to pursue his interests in foxhunting, golf and cricket. His Jewish father and Anglican mother separated when Sassoon was five, and he was denied the family fortunes by the disowning of his father by Sassoon's grandmother. Nevertheless, he was wealthy enough to be able to live without a profession.

Sassoon enlisted into Sussex Yeomanry as a trooper two days before the outbreak of the First World War, and was commissioned into the Royal Welch Fusiliers following a riding accident. His division was soon sent to France, and shortly afterwards Sassoon was to suffer personal experiences that were to affect him deeply. Being an innocent, Sassoon's reaction to the realities of the war were all the more bitter and violent - both his reaction through his poetry and his reaction on the battlefield (where, after the death of fellow officer David Thomas and his brother Hamo at Gallipoli, Sassoon earned the nickname "Mad Jack" for his near-suicidal exploits against the German lines). Sassoon also showed his innocence by going public with his protest against the war (as he grew to see that insensitive political leadership was the greater enemy than the Germans). his friend and fellow poet Robert Graves convinced the review board that Sassoon was suffering from shell-shock and he was sent instead to the military hospital at Craiglockhart where he met and influenced Wilfred Owen.

Sassoon is a key figure in the study of the poetry of the Great War: he brought with him to the war the idyllic pastoral background; he began by writing war poetry reminiscent of Rupert Brooke; he mingled with such war poets as Robert Graves and Edmund Blunden; he spoke out publicly against the war (and yet returned to it); he influenced and mentored the then unknown Wilfred Owen; he spent thirty years reflecting on the war through his memoirs; and at last he found peace in his religious faith.

He did not serve in the Second World War, but lived with his wife Hester Gatty (whom he married in 1933) and their son George, in Wiltshire. Although the marriage ended in 1945, Sassoon lived on in his Wiltshire home at Heytesbury House until he died in 1967, at the age of 80.

Siegfried Sassoon's epigrammatic and satirical war poetry brought much of the living horror of the Great War to public notice. His style is, in many ways, simple - unburdened by lyrical verse and easy to read and understand. his anger was against those at home who simply failed to comprehend the hell that was being lived through each day by the men at the front.