Howard Sisson was ordained a Congregationalist minister in the Congregationalist Church of Aroostook, Maine, on October 4, 1910. His early ministry was in Washington County, Maine, in 1910, Machiasport and East Machias from 1911 to 1914, and Island Falls, Maine, from 1914 until he entered military service in 1917. He was commissioned a U.S. Army chaplain, serving with the American Expeditionary Forces in France where he was wounded and cited for gallantry. In 1919 he addressed the U.S. Senate on the subject of proper respect and burial of the war dead. He continued to serve as a military chaplain until his retirement from active duty as a first lieutenant, on June 14, 1928.