The fantasy scholar Colin Manlove dismisses Lindsay‘s war experience in one sentence by saying, confusingly, that he worked for the insurance brokers Price Forbes and “remained there until 1918, advancing to accountant and then confidential clerk, absent only for war service in London with the Grenadier Guards (1916–18).”
According to Bernard Sellin, Lindsay enlisted and was detailed to join the Scots Guards; instead he found an assignment with the Grenadier Guards, but working in an administrative capacity in an office in Birdcage Walk, close to Whitehall. The Wikipedia entry, however, claims that he moved from the Grenadier Guards to the Royal Army Pay Corps, which sounds plausible. I shall endeavour to reconcile these points when I can.
While working at Birdcage Walk, and frequenting a nearby literary club, he met Jacqueline, and, a few months later, married her, in December 1916. He had another two years of military service. As soon as he was demobbed he resigned from Price Forbes (who had presumably been keeping the post open for him) and planned the life of a writer.
Sellin tells us that Lindsay always hated military life, above all the discipline. He made this comment in his journal: “After a course of years, every soldier acquires more of less insanity; the result of his moral training” (Sellin 18).
For biographical fact I have relied mostly on Bernard Sellin, The Life and Works of David Lindsay, transl. Kenneth Gunnell (Cambridge: Cambridge University Press, 1981), preferring it over the entry in the Oxford Dictionary of National Biography by Manlove.
Science Fiction and Fantasy Writers in the Great War 