He was born in Colborne Township, Huron County, Canada West, the son of Robert Proudfoot, an immigrant from Scotland. He was educated in Goderich, studied law at Osgoode Hall and was called to the bar in 1880. Proudfoot set up practice in Goderich. He married Marion F. Dickson in 1886. In 1902, he was named King's Counsel.
In 1908 he was elected to the Legislative Assembly of Ontario as a Liberal. He was re-elected in 1911 and 1914. In 1917, he was chosen leader of the Liberal Party and, as such, became Leader of the Opposition in the legislature. The 1919 election saw the Liberals and their allies drop from 30 seats to 27 with Proudfoot himself defeated in his riding of Huron Centre by the Labour candidate. No longer having a seat in the legislature, Proudfoot was challenged as Liberal leader at the party's first leadership convention and was replaced by Hartley Dewart. He then became a member of the Senate of Canada where he sat until his death in 1922.
- William Proudfoot – Parliament of Canada biography
- A cyclopædia of Canadian biography : brief biographies of persons ..., HW Charlesworth (1919)
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