William Dennes Mahan was born July 27, 1824 in Pittsylvania County, Virginia to William P. and Permelia Mahan. In 1837 his family moved to Missouri, and by 1845, he was living in Saline County, Missouri where he was serving as a minister in the Cumberland Presbyterian Church. Mahan married Martha R. Johnston in 1850, and together they had two daughters. Mahan was living and preaching in Boonville in 1872.Minutes of the General Assembly of the Cumberland Presbyterian Church, 1872, 123. Mahan died October 19, 1906 in Boonville and is buried there in Walnut Grove Cemetery.
