Just after the start of Lt. Gen. Ulysses S. Grant's Overland Campaign in 1864, Ledlie transferred to the Army of the Potomac, commanding a brigade in Maj. Gen. Ambrose Burnside's IX Corps.

On July 30, 1864, they detonated the explosives, creating a crater some 135 feet in diameter that remains visible today.

The Union plan was to exploit the explosion by sending well-rehearsed African-American troops of Edward Ferrero's division into the gap and driving for critical objectives deep in the Confederate rear area.

[2] Burnside, despondent at the change in plans, resorted to a lottery to select a replacement division.

Most damning for Ledlie's reputation was the fact that he did not lead, or even accompany, his men into battle, and a few weeks earlier, during the attacks on Confederate entrenchments at Cold Harbor, he had run and hidden for cover, an event that the enlisted men did not forget, but which managed to escape Burnside's attention.

During the Battle of the Crater, Ledlie and Ferrero were observed behind the lines in a bunker, drinking liquor.

Ledlie died in New Brighton, Staten Island in 1882 and is buried in Forest Hill Cemetery in Utica.