The 3rd Brigade, with Sinclair-MacLagan still in command, participated in the Battles of Pozières and Mouquet Farm from July to September 1916. He left his brigade in December 1916 to become commander of the AIF depots in England. He was appointed a Companion of the Order of the Bath in February 1917 for his war service to date. He returned to the Western Front in July 1917 when the commander of the 4th Division, Major General William Holmes, was killed shortly after the Battle of Messines. Sinclair-MacLagan was to take over command of the division, which he would lead for the remainder of the war.