During his time on the bench, Johnson was a noted Unionist because of his decision to strike down a militia oath to South Carolina and his view that a violation of a law of the United States was a violation of the law of South Carolina.

The General Assembly elected Johnson as Governor of South Carolina in 1846 for a two-year term.

Much discussed was the Wilmot Proviso which would have outlawed slavery in the territory acquired from Mexico as a result of the war and it helped to further push the state towards the brink of secession.

A Unionist would not become Governor of South Carolina again until the end of the Civil War when Benjamin Franklin Perry was appointed by President Andrew Johnson.

After his term as governor, Johnson returned to Upstate South Carolina where he died on January 7, 1855.