At the time of Grant's enlistment, the main contingent of the NZEF, which had formed the New Zealand and Australian Division, was engaged in the Gallipoli Campaign. After initial training, he embarked for the Middle East in October 1915 with the 7th Reinforcements. He joined the 1st Battalion, Wellington Regiment of the New Zealand Division, then being formed in Egypt in the aftermath of the evacuation from the Gallipoli Peninsula. It embarked for the Western Front in March 1916. Grant served on active duty throughout 1916 and into 1917, during which time his battalion fought in a number of battles, including the Battle of Flers–Courcelette, where about a quarter of its strength became casualties. It also fought in the Battle of Messines, where it was involved in the encirclement of Messines village and captured 200 German soldiers. During its time in the battle, it suffered over 400 killed and wounded. In June 1917, he was promoted to corporal. A few months later, his battalion was one of the leading units in the Battle of Broodseinde. It missed the First Battle of Passchendaele that followed a few days afterward and avoided the heavy casualties that befell the units involved. In early 1918 he was made a sergeant.