His accession to the presidency upon Lincoln's death in April 1865 began a period of unusually contentious relations between the legislative and executive branches, as Johnson vetoed Reconstruction legislation, fired Secretary of War Edwin Stanton in violation of the Tenure of Office Act, and ultimately became the first president in the nation's history to face an impeachment trial. The lengthy and contentious trial ended when, on May 16 and 26, the Senate voted on three of the several impeachment articles brought against Johnson. Each time the tally fell one vote short of the two-thirds majority that the Constitution requires for conviction.
Johnson was not renominated for the presidency in 1868 but returned to the Senate in 1875, serving until his death on July 31 of that year.
For further reading: Benedict, Michael Les. The Impeachment and Trial of Andrew Johnso (New York: W.W. Norton & Co., 1973)
Trefousse, Hans L. Impeachment of a President: Andrew Johnson, the Blacks, and Reconstruction (Knoxville: University of Tennessee Press, 1975)