William C. Rein attended high school in Stafford County, Kansas and graduated in 1972 with an Associate of Arts degree from Hutchinson Community Junior College. He went on to earn a Bachelor of Arts in 1974 and a Juris Doctorate in 1977, both from Washburn University in Topeka, Kansas; he later in 2015 completed a masters of arts in English Bible from Maranatha Baptist University.

Rein worked as a private lawyer before becoming legal counsel for Larned State Hospital in Larned, Kansas in 1980. Four years later he became senior counsel in the Kansas Department of Social and Rehabilitation Services' (SRS) Mental Health and Retardation Services division. Rein remained at SRS from 1984 to 1987, when he moved to the Kansas Department of Health and Environment's (KDHE) Bureau of Adult and Child Care to serve as Director of Hospital and Medical Programs. While at SRS in the 1980s Rein drafted legislation for mental health care reform in Kansas.

In 1993, Rein became Director and Chief Presiding Officer in the KDHE's Administrative Appeals Section. After ten years Rein left KDHE and became staff counsel at the Kansas Department of Aging. After opening a private law practice for a few years in the 2000s, Rein has since held several other roles in state government, including chief counsel for the Kansas Department on Aging (later renamed as the Kansas Department for Aging and Disability Services) and superintendent of Larned State Hospital.