During the subsequent Siege of Petersburg, Grindlay again led his regiment through a series of battles, beginning with the Second Battle of Petersburg in June 1864 and the Battle of Globe Tavern in August. At the Battle of Peebles' Farm, the commander of 1st Brigade, 2nd Division, V Corps, of which the 146th New York was a part, was severely wounded. The injured officer, Elwell Stephen Otis, was carried from the field and Grindlay took over his position. Grindlay's tenure as brigade commander lasted only two days, from October 1, to October 3, before a replacement was found. He reverted to his previous position as leader of the 146th, and saw the regiment through several more battles from late 1864 into early 1865. He led his men at the Battle of Boydton Plank Road in late October, and, after being promoted to lieutenant colonel, at Hatcher's Run in early February.