William Aberhart was born on a farm near Kippen, Ontario on December 30, 1878 to William Aberhart Senior and Louisa Pepper. He attended Chatham Business College and received teacher training at the Mitchell Model School and the Ontario Normal School in Hamilton, Ontario. He also obtained a Bachelor of Arts degree at Queen's University in Kingston, Ontario in 1911. He taught in several schools in southern Ontario, becoming principal of the Central Public School in Brantford Ontario, before moving to Calgary in 1910.

During his time in Brantford, Aberhart also preached in local churches and conducted Bible study classes. Although he grew up in a nominally Presbyterian household, Aberhart and his wife entered the Baptist faith when residing in Calgary. Between 1910 and 1915, Aberhart was principal of three public schools in Calgary: Alexandra, Mount Royal and King Edward. In 1915, he was appointed principal of Crescent Heights High School, a position that he held for twenty years. While in Calgary, he continued to preach in a number of churches and also held Bible study classes. In 1918 he founded the Calgary Prophetic Bible Conference to promote Bible study in Calgary. His sermons and teachings were so popular that the Palace Theatre had to be rented to accommodate all who wished to participate. In 1925, he started broadcasting Sunday afternoon lectures on the radio called the "Back to the Bible Hour". These biblical lectures drew listeners from across the Canadian prairies and the adjacent U.S. states. In 1927 he was appointed Dean of the newly organized Calgary Prophetic Bible Institute. This institute served as a centre of worship and biblical studies, and also produced Aberhart's radio broadcasts. During this time, he was often known as "Bible Bill," in reference to his religious preaching.

Aberhart became interested in politics in the 1930s as a result of the Great Depression. He particularly was interested in the 'social credit' theories of Scottish engineer Major C.H. Douglas which addressed the "discrepancy between the costs of production and the purchasing power of individuals." He founded the Social Credit League and began lobbying the Alberta government, held by the United Farmers of Alberta, to adopt some of the social credit policies. When this attempt failed, Aberhart organized Alberta's Social Credit League and the party won the 1935 Alberta provincial election by a landslide. Aberhart himself had not been a candidate in the election, but because he was the leader of the Social Credit League, he was proclaimed Premier of Alberta. He won a by-election in the electoral district of Okotoks-High River two months after being proclaimed Premier. From 1935-1943, Aberhart held the portfolios of Minister of Education and Attorney General. After the general election of 1940, Aberhart represented the multi-member electoral district of Calgary. During his tenure as Premier, Aberhart and the Social Credit government were successful in legislating its "prosperity certificate" program and also in making changes to Alberta's educational system and labour laws. They also established oil and gas conservation practices and provincial marketing boards. However, the Social Credit policies were not fully realized - attempts to change legislation pertaining to the administration of banks or operation of the newspapers in the province were unsuccessful. Although three bills were passed to amend legislation, the Supreme Court of Canada and the Judicial Committee of the Privy Council ruled these bills unconstitutional.

William Aberhart married Jessie Flatt of Galt, Ontario in 1902 and had two daughters, Ola Janet and Khona Louise. He died suddenly on May 23, 1943 while in Vancouver, British Columbia. A high school in Calgary and a long-term care facility in Edmonton are named in his honour.