William was the youngest of six children born to Charles and Mary B. Linn Barnum. His siblings were Martha (m. Issac Ballinger), Putnam, Frank Mitchell, Lida (Lydia) (m. Wallace Hackett, and Ida (m. Alfred Bryant Miller). His mother was married previously to Hugh Montgomery. William had three half siblings from that union; Sarah Montgomery (m. Charles Harry), Jennie and infant male Montgomery. His grandparents were Thomas and Rachel (Leyman) Linn. Great grandparents were Andrew and Martha (Polk) Linn and Michael and Rachel (Neal) Leyman. On August 20, 1900 he married Rosa Belle Morris at Pontiac, Livingston,Illinois. Alex Smith, Minister of the Gospel from Pontiac was the officiant and witnesses were Mrs. Ida Miller and her husband A. B. Miller. They later divorced and William moved to Bay City, Michigan. William and Rosa had 5 children; Mattie (m. Albert Meyer), Bloyce William, Ruth (m. Earnest Floyd Degenhart), Dephane (m. unknown Fisher) and Harriett (m. John Parker).