He studied at the private law school of Judge William T. Gould in Augusta, Georgia and was admitted to the bar.

In 1844, Johnson moved to the state capitol, Milledgeville, where he continued to practice law.

In 1844 he was a presidential elector, and cast his ballot for James K. Polk and George M. Dallas.

He ran unsuccessfully for governor in 1847, and lost the Democratic nomination to George W. Towns; Towns won the general election, and in 1848 he appointed Johnson to the United States Senate seat vacated by the resignation of Walter T. Colquitt.

During the 1856 presidential campaign, Johnson declared that a Fremont victory would be grounds for secession.

To try to recapture some southern votes, Johnson was chosen as the northern Democrats' nominee as the running mate of presidential candidate Stephen A.

When it became clear that Georgia would secede, however, he acquiesced out of loyalty to his state and served as a senator of the Second Confederate Congress from 1862 to the end of the war in 1865.