King served as the Postmaster General for only 21 days in 1861 (February 12 to March 4), the shortest tenure in the office's history.

On retiring from office, he remained in Washington, D.C., during the Civil War, serving on a board of commissioners to carry into execution the emancipation law in the District of Columbia.

After his retirement from office, King practiced in Washington as an attorney before the executive department and international commissions.

He was active in procuring the passage of three acts in 1874, 1879, and 1885 respectively, requiring the use of the official "penalty envelope," which has secured a large saving to the government.

In 1881, he purchased land in West Newton, MA and built a house there on the corner of Temple and Sterling Streets which he and his wife Isabelle used as a summer residence, near the home of their son, Henry Franklin King.