 While still at Harvard, he wrote a book of poetry, "Castles in Spain", and had two poems published in the Harvard publication, "Made to Order".Leffingwell served in World War I in France as a member of the Intelligence Department, 108th Infantry Regiment, 27th Division, under the command of General John F. O'Ryan.Leffingwell was married to Helen Lillian Urie; the couple had two daughters, Elisabeth and Joan Jackson.