In August 1914 Scott Moncrieff was given a commission in the KIngs Own Scottish Borderers and served with the 2nd Battalion on the Western Front from 1914 to 1917. He was converted to Catholicism at the front in 1915. On 23 April 1917, while he was leading the 1st Battalion in the Battle of Arras he was seriously wounded by an exploding shell. He avoided amputation, but the injuries to his left leg disqualified him from further active service and left him permanently lame. After his release from hospital in March 1918 Scott Moncrieff worked at the War Office in Whitehall. He supplemented his income by writing reviews for the New Witness, a literary magazine edited by G K Chesterton.  At Robert Graves wedding in January 1918, Scott Moncrieff met the war poet Wilfred Owen in whose work he took a keen interest. Through his role at the War Office Scott Moncrieff attempted to secure Owen a home posting and, according to Owen's biographer Dominic Hibberd, the evidence suggests a "brief sexual relationship that somehow failed"

After Owen's death Scott Moncrieff's failure to secure a "safe" posting for Owen was viewed with suspicion by Owen's friends, including Osbert Sitwell and Siegfried Sassoon.  During the 1920s Scott Moncrieff maintained a rancorous rivalry with Sitwell, who depicted him unflatteringly as "Mr X" in All at Sea.  Scott Moncrieff responded with the pamphlet "The Strange and Striking Adventure of Four Authors in Search of a Character, 1926", a satire on the Sitwell family.