William Smith Leonard was born in Dublin, New Hampshire, on October 13, 1832. He was the son of Rev. Levi Washburn Leonard (1790-1864), a Congregational minister, and Elizabeth Morrison Smith (d. 1848). Leonard attended Phillips Exeter Academy and Dartmouth College, receiving degrees from the latter in 1856 (Classics) and in 1860 (Medicine). He briefly practiced medicine in Amesbury, Massachusetts, before settling in Hinsdale, New Hampshire, and going into practice with Frederic Boyden. Eventually, he took over Boyden’s large medical practice. On April 30, 1861, Leonard married Martha (Mattie) E. Greenwood, also of Dublin, New Hampshire, and they had seven children.
In addition to his career as a physician, Leonard was very active in civic life. He was a member of the New Hampshire Medical Society and was twice chosen as a delegate to Dartmouth Medical School. He also held roles in the Hinsdale Board of Education, the Hinsdale Public Library Committee, and the Board of Examining Surgeons at Keene. Leonard died at his home in Hinsdale on June 28, 1902.