As a young congressman who voted against a declaration of war in 1917, South Dakota's Johnson could not live with approving appropriations to support "sending other women's sons into war." So, although he was exempted from service, Johnson voluntarily enlisted in the Army, engaged in battle and was severely wounded. He survived and resumed his congressional career, where he authored the resolution to incorporate The American Legion.
For his years of service, Johnson posthumously received the Legion's Distinguished Service Medal in 1953. He was recognized by Tom Miller, who served with Johnson during World War I and presided over the Paris caucus that led to the birth of the Legion.
In his tribute to Johnson, Miller remarked, "After a year of hospitalization, he again took his seat in Congress, to which he had been re-elected without opposition by his constituents in South Dakota in November 1918, at the time he was lying hospitalized in France."
Accepting the award on behalf of his family, Johnson's son, Navy Capt. Harlan T. Johnson, remarked, "Dad was very proud of what he was able to accomplish, both for national defense and particularly for the veterans who suffered in the war, the maimed and the injured. He was very proud of his association with the Legion. The Legion supported him, and he supported the Legion."When the House created the Committee on Veterans Affairs in the 1920s, Johnson was selected as its first chairman. He served in the role for 10 years, until the Republicans lost the majority in the House. He also served as chairman of the Committee on Expenditures in the Department of War and of the Committee on World War Veterans' Legislation.
For his valor in World War I, when he refused an ambulance ride until his comrades were treated, Johnson earned the Distinguished Service Cross from the United States and the Croix de Guerre from France.
A lawyer by trade, Johnson retired from the House in 1932. He continued to practice law in Washington, D.C., until his death in 1939, at 56.