Levesque was first elected to what is now called the Quebec National Assembly in the riding of Bonaventure in 1956 and sat in the legislature continuously until the end of his life.
After the defeat of the Bourassa government in 1976, Levesque served as Leader of the opposition until 1979, while leaders Robert Bourassa and then Claude Ryan were without parliamentary seats.
Levesque was noted for his fierce opposition to what was introduced as Bill 1, the Charter of the French Language; his procedural wrangling meant it had to be eventually reintroduced as Bill 101.
Levesque was also interim leader of the party between Bourassa's resignation and the election of Ryan.
In the second Bourassa government, elected in 1985, Levesque served as minister of finance, a position he held until his death in 1993 at the age of 67.