Siegfried Loraine Sassoon was born on 8th September 1886, at a house named Weirleigh in the village of Matfield, Kent. His parents were Alfred Sassoon, who belonged to a notable family of Jewish merchant traders, and Theresa Thornycroft, a member of an equally notable family of artists and sculptors. Sassoon was the second of the couple’s three sons. Alfred Sassoon, who had been cut off by his family for marrying outside the faith, left Theresa when the children were still small, and died not long afterwards. The keenly-felt loss of his father was to be a major influence on Siegfried’s development and later actions. Although his poetic ambitions appeared in childhood, Siegfried’s somewhat isolated upbringing (he did not go to school until he was fourteen) and relatively easy circumstances (he had a small private income which ensured he did not need to work) combined to turn him into a lackadaisical young man, by his own admission.

Having dropped out of his degree course at Clare College, Cambridge, and lived for a time in London while vainly attempting to carve himself a literary career, Sassoon managed one minor commercial success with The Daffodil Murderer, a parody of a popular work by John Masefield. Some of his other verse was privately printed, without acclaim, and then came the First World War. Sassoon joined up on the day before war was declared, and went into the Sussex Yeomanry as a mere trooper, in the hope of being able to keep his horse. He saw no action for over a year. It was only when, after being incapacitated in a fall, he determined to start afresh with a commission in the Royal Welch Fusiliers, his military career really began.

During his time at the Western Front, Sassoon became friendly with two younger men who were formative influences on his life. One was the younger poet, Robert Graves, who encouraged Siegfried to look with new eyes on his surroundings. The other was David Thomas, who was tragically killed in 1916. This death, combined with that of his brother Hamo - who was very badly injured at Gallipoli and subsequently died on a hospital ship from the effects of his wound - caused Sassoon great emotional turmoil. 

A sympathetic picture of his suffering is drawn by Graves in Goodbye to All That. While recovering from trench fever at a convalescent hospital in Oxford, Siegfried became friendly with Lady Ottoline Morrell and her circle, including intellectuals and pacifists such as Bertrand Russell. Russell is credited by some with having been the catalyst leading to the issue of the "Soldier’s Declaration", a statement which was printed in some newspapers and read out in the House of Commons in July 1917. Sassoon, in the grip of a "neurosis", had by now thrown the ribbon of his Military Cross medal into the River Mersey and refused to report for duty. His glittering military record ensured that the authorities declined to court-martial him. Instead, he was sent for treatment to Craiglockhart Military Hospital in Edinburgh.

Craiglockhart was an experimental facility, whose remaining buildings are now part of Napier University. The attentions of the psychiatrists who worked there were reserved for officers, of whom Siegfried Sassoon was one and Wilfred Owen another. Owen was several years younger than Sassoon, but had read a recent collection of poetry, The Old Huntsman, in which Siegfried had begun to give an effective voice to the bitterness and pain of the war experiences he shared with so many others. As a result, Owen hero-worshipped Sassoon, and they quickly became fast friends. Both were soon released from Craiglockhart, Sassoon after several months under the care of the kindly Dr William Rivers, whom he came to see as a surrogate father. Sassoon was posted to Palestine, but Owen did not return to active service until the following year.

Sassoon’s final period of service on the Western Front was over almost before it had begun. Having taken part in a daring raid on enemy lines, he rashly removed his helmet and was shot in the head by one of his own men. The wound was not serious, but it put an end to his military service. Ironically, shortly after he returned home for good, Owen went back to the Front and was killed only a week before the end of the war. Sassoon’s most famous poem, "Everyone Sang", was written at about this time; he did not yet know of Owen’s death.

After hostilities ended, Sassoon found it difficult to adjust to civilian life, and went through a period of depression. His war poetry, which had been much in demand for the last year or two, gradually came to seem out of place, and he had to cast around for another outlet. A lecture tour in the USA and a period as literary editor of the Daily Sketch both failed to renew his inspiration, but a longer-term project, an autobiographical "novel", was finally realised in 1928. At first published anonymously, Memoirs of a Fox-Hunting Man swiftly became a best-seller, and established Sassoon as a prose writer of distinction. He followed up the adventures of his alter ego, George Sherston, with two more volumes, Memoirs of an Infantry Officer and Sherston’s Progress, by the end of which it was clear to everyone that he was writing his own life story. During the 1930s and 1940s, he went further and wrote three volumes of autobiography in which he explored himself more deeply and delved into aspects of his life he had omitted from the Sherston trilogy. These works, The Old Century, The Weald of Youth and Siegfried’s Journey, are equally highly regarded.

It was soon after the war that Sassoon, encouraged by his mentor, Robert Ross, had embarked on his first homosexual relationship, with an artist, Gabriel Atkin. Other lovers, notably the aristocratic Stephen Tennant, came his way, but he always hoped to have a son of his own, and his wish came true when he married Hester Gatty, many years his junior, and she gave birth to his only child, George, in 1936. Sadly, the marriage failed, and Hester went to live in the north of Scotland. The Sassoons, thanks to a legacy from Siegfried’s paternal aunt, the wealthy Rachel Beer, had been able to buy Heytesbury House in Wiltshire, where Siegfried continued to live, now something of a recluse. It was during the 1950s that he became friendly with Dennis Silk. Their public school backgrounds and love of cricket were common ground, and Dennis is now the Life President of the Siegfried Sassoon Fellowship, and has spoken many times in public about his friendship with Sassoon. In 2004, he revisited Heytesbury to make a programme for Radio 4, in which recordings of Sassoon reading his own poetry were broadcast for the first time.

In his latter years, Sassoon became religious, something he had never previously been, and he converted to Roman Catholicism in 1957, strongly influenced by figures such as Dame Felicitas Corrigan and Dom Philip Jebb. Most of his later poetry was inspired by this new force in his life. He died on 1st September, 1967, and is buried in the graveyard of St Andrew’s Church, Mells, close to Ronald Knox, whom he greatly admired. It was not until 2001 that a literary society was formed to celebrate the life and work of Siegfried Sassoon. The founding President was Michèle Fry, whose "Counter-Attack" website, now sadly unavailable, attracted thousands of Sassoon enthusiasts in the early years of the Fellowship.