When the Civil War broke out, Johnson joined the
Scandinavian Regiment (15th Wisconsin Infantry Regiment) and was appointed captain of Company B. After the
Battle of Perryville, Johnson was promoted to the rank of major. He took part in the battle of
Murfreesboro and reached the rank of lieutenant colonel in March 1863. The Scandavian Regiment was heavily mauled at the
battle of Chickamauga with approximately 63% killed, wounded, or captured. Colonel
Hans Christian Heg who had commanded the Regiment since its formation was killed and Johnson was captured. Johnson was held in
Libby Prison in Virginia before it was decided in May 1864 to move him to another prison. At
Chesterville, Johnson and two others escaped from the railroad car they were being transported in. After a month, the men were able to rejoin the Union Army at
Strawberry Plains, Tennessee. Johnson was promoted to colonel and took command of the Scandinavian Regiment on July 24, 1864, remaining with the regiment until the expiration of the regiment's service in February 1865. ==Post Civil War==