Who served as the 17th President of the United States following the assassination of Lincoln?

Andrew Johnson served as the 17th President of the United States following the assassination of Abraham Lincoln. Johnson, who was Vice President at the time of Lincoln's death in April 1865, assumed the presidency as mandated by the presidential succession law. His presidency is notable for the period of Reconstruction that followed the Civil War, during which he attempted to implement policies for the reintegration of Southern states into the Union.

His approach to Reconstruction and the relationship with Congress were particularly contentious, leading to significant political conflict, including his impeachment by the House of Representatives. Johnson's policies favored leniency toward the Southern states and did not adequately protect the rights of newly freed African Americans, which created further divisions within the country.

The other choices do not apply to this question. Ulysses S. Grant was the 18th President, Thomas Jefferson was the 3rd President, and James Buchanan was the 15th President. Each of these figures holds significant historical importance in their contexts, but none were in office directly following Lincoln's assassination.