After brief service in the Tennessee Senate, Johnson was elected to the House of Representatives in 1843, where he served five two-year terms. He became governor of Tennessee for four years, and was elected by the legislature to the Senate in 1857. As the conflict grew between the branches of government, Congress passed the Tenure of Office Act restricting Johnson's ability to fire Cabinet officials. He persisted in trying to dismiss Secretary of War Edwin Stanton, but ended up being impeached by the House of Representatives and narrowly avoided conviction in the Senate. He did not win the 1868 Democratic presidential nomination and left office the following year. Although Democratic President Martin Van Buren was defeated by former Ohio senator William Henry Harrison, Johnson was instrumental in keeping Tennessee and Greene County in the Democratic column. He was elected to the Tennessee Senate in 1841, where he served a two-year term. He had achieved financial success in his tailoring business, but sold it to concentrate on politics.