Johnson moved through the chairs, from local alderman to major to state representative and senator, to U.S. congressman to governor of Tennessee to U.S. senator. A radical Jacksonian, he strongly believed that government at every level should interfere as little as possible in people's lives. He also believed that the U.S. Constitution imposed such limitations on the national government. Johnson defended slavery through the 1850s, taking a strong states' rights position.

In 1860 he supported the proslavery Democratic presidential candidate. When the Southern states began to secede, however, Johnson disagreed. He did not believe that secession could be undertaken legally. He fought to keep Tennessee in the Union, refused to resign from Congress, and was the only Southern senator to remain loyal to the Union. When Union forces occupied part of Tennessee in 1862, Johnson was appointed by Abraham Lincoln as military governor.

Johnson became Lincoln's running mate in the 1864 campaign. Succeeding to the presidency after Lincoln's assassination, Johnson tried to restore the Union with as little change as possible--other than slavery. Johnson pardoned nearly all Southerners for their part in the rebellion and urged Congress to allow representatives who passed loyalty tests to take their old seats. Congress refused and Johnson split with the Republican Party. Like most Southerners, Johnson believed whites should maintain firm control over society and government.

He believed in states' rights and believed Congress had no power to interfere in Southern states' internal affairs. He then opposed Republican legislation to protect the rights of ex-slaves. The struggle between president and Congress came to a head in 1868. Johnson defied the Tenure of Office Act, which Congress passed over his veto and he fired Secretary of War Edwin M. Stanton. Congress responded by impeaching the president. After a trial that lasted for several months, the Senate found Johnson not guilty. His power had been broken, however, and he returned to Tennessee at the end of his term.