After arriving in Missouri he and his brother both offered their services to General Curtis. Johnson became a mail agent and marched with the Union army to Helena. Johnson later served as a volunteer aide to the staff of General Schofield, commander of the Army of the Frontier. In November 1862 he was given the authority by General Schofield to raise a regiment of loyal Arkansas troops. Upon receiving the order, he marched with the army during the Prairie Grove campaign. He opened up a recruiting office in Fayetteville.