Known as Billy to friends and family, Foster was born in Bristol, England. He studied engineering at Wycliffe College before he emigrated to British Columbia in 1894, where he became involved in the lucrative lumber business. He served with the Canadian Pacific Railway as a superintendent and police magistrate in Revelstoke, manager for the Globe Lumber Company on Vancouver Island, President of the Conservative Party of British Columbia, provincial Member of the Legislative Assembly, and Minister of Public Works prior to the Great War. Foster was an avid mountaineer and was on the first expeditions to climb Mount Robson and Canada's highest peak, Mount Logan.