The division crossed the James River on 28 March and moved southwards through the night to arrive in front of the Confederate fortifications on 29 March. The division participated in the Third Battle of Petersburg, during which it attacked Forts Gregg and Alexander on 2 April. The regiment attacked Fort Gregg on the right of the brigade, commanded by Brevet Brigadier General Thomas O. Osborn. Advancing in line of battle at the head of the division, the 199th was to the left of the 67th Ohio and the 39th Illinois. It counterattacked the Church Road with the brigade at 09:00, forcing back Confederate troops from a salient that they had gained in a counterattack earlier that day. At 13:00 the brigade was committed to the attack on Fort Gregg, advancing through a hail of fire. Briscoe was hit in the leg within the last 300 yards to the fort, but continued onward with others from the regiment into the ditch in front of the fort, partially filled with water. There, First Lieutenant Oliver Sproul of Company H planted the colors of the 39th Illinois in the parapet of the fort after the 39th's color bearer was shot. Along with other Union troops, the men of the regiment advanced into the fort and captured it. The regiment lost eighteen killed or mortally wounded, and 91 wounded; among the latter were six officers, including Briscoe and two company commanders. Briscoe was made a brevet brigadier general for his gallantry, and was promoted to command the brigade; he was replaced by Lieutenant Colonel Robert P. Hughes.