The United States entered the World War I on April 6, 1917. The Nation would need to train an army in order to conduct this war. In September of 1917, Bishop Muldoon sent Father Joseph Lonergan to the new U. S. Army training base, known as Camp Grant, south of the city of Rockford, Ill. Father Lonergan was the bishop's personal representative to Camp Grant. This experience would change and shape the career of the young priest for a lifetime. Father Lonergan began conducting Catholic services and Masses for the army cantonment of over 50,000 Dough Boys.
Camp Grant was the only World War I base to organize a full Army division. “Father Joe,” as the soldiers called him, provided leadership and spiritual comfort to the men. Chaplain Lonergan would accompany the 86th Division to France for combat in the trenches of the World War. Father Lonergan was soon assigned to the 20th Engineers in the Vosges Mountains near the southern edge of the Argonne Forest. The 86th Division soldiers were scattered from Alsace to Bordeaux with 20,000 men taking part in some very bloody fighting. Chaplain Joe was in the trenches and under fire with his soldiers. He helped give courage and compassion to the living and comfort and absolution to those who would not survive. Father Joe also helped to write the last letters for several dying soldiers at the front. As promised, he did bring home those last soldier letters to their families.
In April of 1919, Father Lonergan was promoted to the rank of captain and assigned to division headquarters serving under General Martin. In 1921, he returned to parish work at St. Mary Church in Durand, Ill. While serving there, Father joined the newly-formed American Legion of veterans of World War I and served as department chaplain for Northern Illinois. His duties would take him around the state of Illinois to speak on veterans' issues, service, assistance for widows and children and the needs of returning servicemen. The American Legion held their national convention at St. Paul, Minn. in 1924. Father Joseph Lonergan was elected to the post of national chaplain of The American Legion for 1924 to 1925. He would remain very active in speaking at Legion posts and events around the nation. Chaplain Lonergan would say, "A good Legion post is like a good parish church-where everyone feels both at home and will work together both to produce a good unit and more loyal people."