He participated in the Second World War French Resistance movement.
He was arrested by the German forces in August 1944 but managed to escape.
After the Liberation, he became a general inspector at the Ministry of Defence.
Under the Fourth Republic, he held ministerial posts numerous times (11 posts in 10 years) and was a member of the Christian-Democratic Popular Republican Movement (MRP).
He became senator from Seine-Maritime in 1959 and was president of the MRP from 1963 to 1965.
In 1965, he ran in the presidential election as a center-right candidate.
He was supported by Paul Reynaud.
He advocated modernity and European integration and declared to represent a third way between Gaullism on the one hand and the Socialist and Communist Left on the other hand.
His "modern-style" campaign and dashing smile had some journalists nickname him "the French Kennedy".
Lecanuet obtained 3,777,120 votes (15.6%) in the election's first round, forcing Charles de Gaulle to compete in a second round against François Mitterrand.
He replaced the ageing MRP by the Democratic Centre, integrating the liberal-conservative National Centre of Independents and Peasants.
In 1972, he founded the Reforming Movement with Jean-Jacques Servan-Schreiber.
During the French legislative elections of 1973, Lecanuet negotiated the withdrawal of candidates with Pierre Messmer to ensure the success of the majority.
From 1979 to 1988, he was a Deputy in the European Parliament and, as Senator for Seine-Maritime, the French Senate's chairman of the Foreign Affairs and Armed Forces Commission, a post that he had already held between 1971 and 1973.
In 1986 at the beginning of the first period of "cohabitation" in modern French politics (a President and Prime Minister from opposing parties sharing power) Chirac nominated Lecanuet as Foreign Minister, but President François Mitterrand vetoed the appointment, along with some of Chirac's other nominees.In 1968, he was elected Mayor of Rouen, a position he held until his death in 1993.