Williams received a brevet to the rank of captain for his conduct at the Battle of Cerro Gordo as a first lieutenant.

[1] President Lincoln nominated Williams for the promotion on December 21, 1861, and the U.S. Senate confirmed the appointment on July 17, 1862.

[3] From August 20, 1861, to March 1864, Williams was assistant adjutant general of the Army of the Potomac, responsible for the routine drafting of orders, correspondence, and reports.

Williams was called before the United States Congress Joint Committee on the Conduct of the War to testify about the Battle of Gettysburg.

When Grant decided to recommend surrender to Robert E. Lee during the Appomattox Campaign, it was Williams who took the message to the Confederate lines.

[6] Although slated to begin service as assistant adjutant general of the Military Division of the Atlantic, Williams became ill later that month and left for Boston, Massachusetts.