He rose to power by mobilizing the Francophone opposition to the execution of Louis Riel, denouncing it as a betrayal by John A. Macdonald's Conservative government.

Mercier studied at the Jesuit College Sainte-Marie in Montreal, and was called to the Bar of Quebec in April 1865.

[1] In 1879, Mercier was appointed Solicitor General of Quebec in the Cabinet of Premier Henri-Gustave Joly de Lotbinière and served in that position for less than a year.

[1] Seizing the opportunity to build a coalition with dissident Conservatives, Mercier revived the "Parti National" name for the 1886 Quebec provincial election, and won a majority of seats.

The Conservatives, reduced to a minority in the Legislative Assembly, clung to power for a few more months, but Mercier became Premier of Quebec in 1887.

Seeing provincial autonomy as the political expression of Quebec nationalism, he collaborated with Ontario Premier Oliver Mowat to roll back federal centralism.

He promoted contacts with Francophones in other parts of North America outside of Quebec including Western Canada and New England.

On December 16, 1891, he was dismissed by Lieutenant Governor Auguste-Réal Angers after a report concluded that his government had diverted public funds.