Johnson worked as a tailor in his own shop in Greeneville, Tennessee. By the age of twenty-two, he was elected the mayor of Greeneville. He served in that capacity for four years until 1833. In 1835, Johnson was elected to the Tennessee House of Representatives and by 1841 he became a Tennessee state senator.

Move to National Politics

Having done well for himself in the state legislature, Johnson moved into national politics. In 1843, he was elected to be a U.S. representative for his district. He remained a representative until he was elected as governor of Tennessee in 1853. By 1857, he was elected to be a U.S. senator from Tennessee. While in his legislative roles, he worked hard to defend the rights of the common man, even advocating the Homestead Act, which would give free farms to the poor. He also defended slavery and supported the Fugitive Slave Act, which was part of the Compromise of 1850. Despite this, he was the only Southern senator who did not agree with secession and retain his seat upon the secession of his state. In the South, he was viewed as a traitor. Johnson saw not only the secessionists but also the abolitionists as enemies of the Union.

Military Governor of Tennessee

Due to Johnson's loyalty to the Union, Abraham Lincoln made him the military governor of Tennessee in 1862. When Lincoln ran for reelection in 1864, he chose Johnson as his vice president to balance the ticket, as Johnson was a southerner who was also pro-Union. They won a huge victory against the Democrats.

The conspirators who killed Lincoln had also planned to kill Johnson. However, the conspirator who was to kill him did not follow through, and Johnson became president following Abraham Lincoln's assassination on April 15, 1865.