Siegfried was educated at Marlborough Grammar School and later attended Clare College at Cambridge. His main interests were hunting and poetry. He was an undergraduate at Clare from 1905 to 1907 and was later made an Honorary Fellow in 1953. Five years after leaving Clare College he wrote a parody to the poem "The Everlasting Mercy" by John Masefield. Mr. Masefield was so impressed by the work that he hailed Sassoon as "one of England's most brilliant rising stars...". Sassoon's parody titled "The Daffodil Murderer" was written in December of 1912.

Sassoon enlisted in the military at the age of 28 just before the draft and was eventually assigned to the Royal Welch Fusiliers. There he met and befriended the writer Robert Graves.

The war was hard on Siegfried and his family. Early in the war Sassoon's brother Hamo was mortally wounded at Gallipoli. Hamo was later buried at sea. Siegfried took vengence for his brother's death by involving himself in brave, sometimes suicidal deeds against the Germans. A short leave from the front helped to calm him and later as the war dragged on, he experienced a sense of total disgust with the conflict. This distain would work its way into his poetry. During a spell of convalescence in which he was treated for shell shock at Craiglockhart Hospital in Edinburgh, Scotland he met and befriended the poet, Wilfred Owen who was being treated for the same ailment. He also met, befriended, and had a profound effect on the opinions of his doctor W.H.R. Rivers regarding war.

Since Wilfred Owen had not yet become a published author when he was killed in action his poetry was unknown to the public. Sadly Owen was killed one week prior to armastice.

After the war Siegfried insisted on working as editor and publisher of Owen's works. Within that same year Sassoon discovered that he was too intimately involved with both Wilfred Owen and The Great War to maintain the objectivity required to serve as both editor and publisher. Another year passed before Sassoon found an editor for Owen's poetry and "Anthem for Doomed Youth" was published shortly after.

Sassoon's early war poetry gives the reader a sense of war as a noble enterprise; his later war poetry attacks the entire nature of war and those who profit by it.[I1] [E1] [E2][S1] After the war he also published a series of fictional autobiographies in which he recounts his life before ( Memoirs of a Fox Hunting Man ), during ( Memoirs of an Infantry Officer ), and after ( Sherston's Progress ) the Great War. Some years after the war he wrote autobiographical prose and poetry which did not relate to the war.

Of his war poetry, "The Old Huntsman" appeared in early 1918 singing the nobility of war; "Counter-Attack" appeared in late 1918 revealing the shocking brutality and pointlessness of war.

In December of 1933 Siegfried married Hester Gatty after a short engagement. Their only child, George, was born in 1936. Sometime after the end of the second world war Siegfried and Hester divorced.