On April 16, 1865, the U.S. War Department issued General Orders, No. 67, announcing that Andrew Johnson had assumed the office of President of the United States.
General Orders, No. 67
War Department, Adjutant General’s Office
Washington, April 16, 1865
It is hereby announced to the Armies of the United States that on Saturday, the 15th day of April, 1865, by reason of the death of Abraham Lincoln, the office of President of the United States devolved upon Andrew Johnson, Vice-President, who on the same day took the official oath prescribed for the President and entered upon the duties of that office.
By Command of Lieutenant-General Grant:
Assistant Adjutant General.