After his defeat, he practiced law in Donaldsonville, Louisiana, located on the south bank of the Mississippi River in the south-central part of the state.

[2] During his term, the legislature moved the state seat of government to Donaldsonville, a compromise location settled on between Anglo-American leaders, who wanted the capital moved from New Orleans to a more northerly location, and French Creoles, who wanted to retain the seat of government within an historically-French area to reflect the state's origins.

He also enjoyed the goodwill of a visit to Louisiana by the American Revolutionary War hero, the French aristocrat Marquis de Lafayette.

Johnson inflamed the conflict again by taking the side of the "Anglos" in a dispute about cotton and sugar cane cultivation.

In 1834 Johnson was elected as a Whig to the U.S. House of Representatives, to fill the vacancy after the resignation of Edward Douglass White, Sr.

[5] That same year, Johnson sold a share of Chatham and would eventually sell the remainder of his land and enslaved people to John R. Thompson in 1851.

In 1844, Johnson was elected to fill the vacant U.S. Senate position of Alexander Porter, who never took the seat due to ill health and died in January 1844.

As senator he supported bills favoring the annexation of Texas, which had become an independent Republic after separating from Mexico.

In 1848 Johnson lost a bid to remain in the Senate to Pierre Soulé, a Jacksonian-Democrat of French Creole descent.

In 1850, he suffered a final political defeat, losing a race for U.S. Representative against Henry Adams Bullard (Whig).

Several years after the United States made the Louisiana Purchase, the Johnsons moved to the Territory of Orleans, in 1809.