This is a series of forty-six journals, mainly covering the period 1915-1959, along with 119 enclosures. The volumes are primarily ruled notebooks rather than printed diaries but do never the less contain many diary entries. Sassoon made an active decision to keep a more discursive written account of his activities during the First World War and the journals prior to that are only used to record sporting engagements.

At first Sassoon treated the volumes as journals using them for a wide variety of other purposes. In addition to making diary entries he drafted poetry (often dating it and stating where it was written); listed extent of literary work completed; made pencil or pen and ink sketches; made preparatory sketches for water colours; made diagrams or maps of the trenches; took notes at military briefing meetings; listed members of his battalion and their fates; noted quotations and transcribed letters; recorded people's contact details; listed monies spent, loaned or gifted; listed locations of nights spent away from home; listed books taken away; from the 1930s made notes on the weather; and pasted in photographs, items of ephemera and press cuttings. Increasingly in the post war period he makes a clearer division in the notebooks he is using; those in this series become more clearly diaries while he keeps separate commonplace books (MS Add.9852/4) and notebooks for drafting poetry (MS Add.9852/6).

From about 1922 diary entries become much more consistent than those in the war journals, although there are still significant gaps and Sassoon often records the activities of three or four days prior under the date heading of the day he is writing on. Sassoon frequently used the volumes in both directions, sometimes keeping a diary from one end and drafting poetry from the other. The volumes are fragile; the smaller volumes were almost certainly carried in a pocket and have suffered considerable mechanical damage as the pages have been turned many times. Sassoon scored out or excised some passages at a later date, sometimes annotating the scoring out.