[6] He was selected by a panel that made a recommendation to Representative Roswell P. Flower, but was subsequently declared ineligible because he did not meet the one-year state residency requirement.

[7] In July 1883, he was appointed a second lieutenant in the United States Army directly from civil life, subject to completion of an examination by a board of officers.

[10] Following the conflict, Johnston was mustered out of the volunteer service and was appointed military governor of Isabela Province on the Philippine island of Luzon, a post which he held from 1901 to 1902.

[19][20] Following the American entry into World War I in April 1917, he was promoted to brigadier general and assigned to command the 180th Infantry Brigade, a unit of the 90th Division.

Under his command, and aided by Colonel Herbert J. Brees as his chief of staff, the 91st Division took part in the St. Mihiel offensive and the Meuse–Argonne offensive, for which he was awarded both the Distinguished Service Cross (DSC) (the second highest military decoration in the United States Armed Forces) along with the Army Distinguished Service Medal, and the Ypres-Lys campaign.

The citation for his DSC reads: The President of the United States of America, authorized by Act of Congress, July 9, 1918, takes pleasure in presenting the Distinguished Service Cross to Major General William H. Johnston, United States Army, for extraordinary heroism in action while serving as Commanding General of the 91st Division, A.E.F., northwest of Verdun, France, from 27 to 30 September 1918.

Major General Johnston repeatedly showed exceptional bravery during the Argonne-Meuse offensive, frequently visiting his front lines under heavy fire from enemy artillery, machine-guns, and snipers, displaying marked coolness and inspiring the members of his command with confidence and determination.

During the Meuse-Argonne offensive, General Johnston commanded with skill and ability, the 91st Division in the difficult advance that resulted in the taking of Epinonville.

Later, in participation with the French, he led his Division with marked distinction in the attack on and capture of the important city of Audenarde in the closing operations of the war in Belgium.

[28] He was buried at Arlington National Cemetery, and the honorary pallbearers included generals Stephen O. Fuqua, Harry L. Gilchrist, Samuel Hof, Lytle Brown, John W. Gulick, and Oscar Westover.