Second Lieutenant Lewis Douglas Lacy was born in Webster Groves, Missouri, on July 11, 1891, the son of Lewis and Maude Lacy. He had one sister, Mabel (b. February 1893). He worked in the automobile insurance industry before enlisting in the United States Army in 1917. During the war, he served as a master engineer. He was stationed on the front line and at general headquarters in France from August 1918 to March 1919. Lacy returned to Webster Groves after the war and married Edith Radford in May 1927.