Grimwade joined the Australian Imperial Force as a lieutenant colonel on 26 August 1915 to command the 4th Field Artillery Brigade, a new unit formed for the 2nd Division.

The 2nd Division Artillery thus missed the Gallipoli campaign and instead began intensive training for France.

Grimwade remained there training his new command for some months, until the 3rd Division Artillery was finally deployed to France on 31 December 1916.

In a quiet sector of the line near Messines the 3rd Division, now part of the Australian Corps, spent the winter preparing for a German Offensive in the Spring.

Grimwade and John Monash clashed with the new Corps Chief of Staff, Major General Brudenell White, over the placement of the artillery, which they felt was too far forward and would be overrun in the event of an enemy offensive.

For the Battle of Amiens on 8 August 1918, Grimwade had control of 9 brigades of field and horse artillery.

In response to a strike by men of the 3rd Division Artillery in January 1919,[2] Grimwade resumed command and listened sympathetically to their troubles, attempted to rectify what he could, and promised to pass the rest on to an equally sympathetic corps commander, Lieutenant General Talbot Hobbs.