Grindlay enlisted from Boonville on September 8, 1862 for a three-year term of service in the Union Army. He joined Company D of the 146th New York Infantry as a captain. In May and June 1864 the regiment took part in General Ulysses S. Grant's Overland Campaign in Virginia. At the Battle of the Wilderness in the first week of May, the 146th took heavy casualties. Both the regimental commander and the second-in-command were killed in action, leaving Grindlay as the most senior officer. He assumed command of the regiment and led it through the Battle of Spotsylvania Court House which immediately followed. Promoted to major, Grindlay led the 146th for the remainder of the campaign, including the battles of North Anna, Totopotomoy Creek, and Cold Harbor.