William H. Harrison was born on October 7, 1896 at Middle River, Maryland. He attended the St. John's College at Annapolis, Maryland, before he reported for Marine Corps active duty with the rank of second lieutenant on May 24, 1917. Harrison was sent to the Basic School for additional officers training and subsequently was attached to the 13th Marine Regiment and sailed for France. He arrived in September 1918, but too late to see combat and spent almost nine months with occupation duties in Brest. Harrison was ordered back to the United States in July 1919 and following the deactivation of the unit, he was attached to the Second Provisional Marine Brigade under Brigadier general Ben H. Fuller. His first expeditionary duties came in October 1919, when he sailed with 2nd Brigade for Santo Domingo to fight rebel forces of General Desiderio Arias.