After a year spent in recruitment and recruit training, Grubb was re-commissioned as colonel and appointed commander of the 37th New Jersey Volunteer Infantry, a 100-day enlistment unit. The new regiment then served in the trenches of Petersburg, Virginia, and in garrison duty until it was mustered out in October 1864. One night after arriving in the trenches, with few arms and little ammunition, the regiment came under fire and took some casualties. Their helplessness in this situation gave rise to the nickname they would bear in the New York Times, "Grubb's Game Chickens". His younger brother, Parker Grubb, serving as the 37th New Jersey's regimental adjutant, died of disease during the regiment's service.