William was born at "Millbrae", Native Valley, near Nairne, South Australia, the son of Richard Mills the younger (1840–1870).
He gained experience as a jackaroo for Robert Browne, manager of Winnininnie Station in the north of the State.
He purchased another property, in the Adelaide Hills, which he named "Sturtbrae", which was later subdivided as Bellevue Heights by his daughters Margaret and May.
[citation needed] He was in 1915 a foundation member of the Farmers and Settlers' Association, and for five years its president.
He married Elizabeth Martha "Lizzie" Champion (1861–1961) on 19 July 1882; they had five sons and four daughters: Most members of the family were buried at Blakiston cemetery.