On July 23, 1863, Meigs was assigned as an engineer to the staff of Brigadier General Benjamin Franklin Kelley. He was also assigned as an engineer and acting aide-de-camp on the staff of Brigadier General (Volunteers) William W. Averell. He engaged in extensive surveying of the area and drew hundreds of topographical maps, and assisted in designing and overseeing the construction of defensive works. He also reconnoitered on behalf of General Averell's troops on several occasions. He participated in the Battle of Rocky Gap on August 26 to 27, 1863, near White Sulphur Springs, West Virginia. Meigs was made Chief Engineer of the Shenandoah Valley for the Department of West Virginia (a Union military district) on November 3, staying in the position until August 17, 1864. On November 6, he participated in the Battle of Droop Mountain in West Virginia, and the pursuit of Confederate forces after the battle from November 6 to 18. Meigs was detached from Kelley's command on November 23.