In 1813 he joined his father's militia unit in the Creek War, returning to Nashville the next year to complete law studies in the firm of Parry Wayne Humphreys.

[3] He advocated legal protection of slavery under the federal constitution, believing that this would prevent "moderate" southerners from being overwhelmed by secessionist Fire-Eaters.

[5] Johnson acted as a campaign manager for presidential candidate James K. Polk at both the Democratic party convention and for the general election.

He urged that telegraph lines not be left in unregulated private hands, concerned that they would ruin the Post Office while enriching those who held preferential information access, but his fellow Democrats were unreceptive.

[2] During the secession crisis he joined the short-lived Union Party that sought to keep Tennessee loyal to the federal government.

He joined in drafting an address that urged the state to remain in the Union while refusing to participate in coercive measures against the Confederacy.