He was elected as the Progressive Conservative Member of Parliament (MP) for Ottawa Centre, and was one of the few francophone MPs in the Tory caucus.
Despite the Tory victory in the 1979 general election, Robert de Cotret lost his seat.
In need of French-Canadian Cabinet ministers, Prime Minister Joe Clark appointed de Cotret to the Senate of Canada and to Cabinet as Minister of Industry, Trade and Commerce in Clark's minority government.
When the government was defeated in a motion of non-confidence, a new election was called for February 18, 1980.
Mulroney appointed Cotret to Cabinet as President of the Treasury Board.
In 1987, Cotret became Minister of Regional Industrial Expansion, and reassumed the Treasury Board portfolio in 1989.
In 1990, he became Minister of the Environment and then Secretary of State for Canada in 1991.
Robert de Cotret retired from Cabinet in January 1993 and did not run in the 1993 election.