Memoirs of an Infantry Officer is actually the second book in a series of fictionalized memoirs about the character George Sherston, essentially a psuedonym for Sassoon himself. Sassoon, who was Jewish and labored under the German name "Siegfried" during a time when this was very inconvenient to him, joked that his life might have been easier if he had been called George. George Sherston is a version of Sassoon whose existence is marginally less complicated, but whose life in the infantry during the First World War borrows very heavily from Sassoon's own. Sassoon based the book upon his war diaries of the period.

Indeed, there could be no doubt that the protagonist of the book is really Sassoon himself. The story may begin as a relatively universal story of a soldier cycling into the war—passing through regulation training, the meeting of friends, and early exposure to the trenches, and peaking with the Battle of the Somme—but the story it tells of objection to the war is entirely Sassoon's. The most memorable section of Memoirs involves Sherston deciding, as Sassoon did, to throw away his military medal in protest against the war, after which he is sent to Craiglockhart Military Hospital and only escapes execution because of the arguments of his close friend, David Cromlech, based on Robert Graves. The book paints an extremely in-depth picture of the inner emotional life of the soldier Sherston, as one might expect, given its source material.

Since its publication, Memoirs has widely been considered one of the most important books of the First World War. However, it can be best understood by contrast to its predecessor, Memoirs of a Fox Hunting Man, in order to truly comprehend the changes war wrought upon its young narrator.