Bourassa studied law but built a reputation as a writer on political affairs. He became mayor of Montebello, Labelle County, Que., in 1890 and represented Labelle in the federal House of Commons as a Liberal (1896–1907). In 1899 he resigned in protest against Canadian support of the imperialist cause in South Africa, but, as the accredited leader of the Nationalist Party, he was reelected in 1900 and 1904.

Bourassa proposed that Canada should become a completely separate nation under the crown, and he cooperated with the Conservatives in opposing U.S. capital investment in his country. From 1908 to 1912 he was a member of the Quebec Legislature, where he opposed the Liberals. He also campaigned against conscription, introduced by the Conservatives in 1917.

In 1925 Bourassa was reelected by Labelle to the federal Parliament as an independent, and the following year he allied with Prime Minister W.L. Mackenzie King in demanding a modification of the role of the governor general, although he declined office in King's new government. He left Parliament in 1935, when his disregard for local political patronage brought him defeat in the general election.