He became president of a Catholic youth group and decided to become a priest, but first served in the French Army during World War I.

[citation needed] During the German occupation, he directed Troyes' medical services and provisioning.

[6] Pope John XXIII, who came to like the "tall, jolly" Lefèbvre while serving as nuncio to France,[6] created him cardinal priest of San Giovanni dei Fiorentini in the consistory of 28 March 1960.

[8] Pope Paul made him a member of the Holy Office in 1964 as part of his campaign to make its leadership less Italian and less curial.

At the final session, on 20 September 1965, he joined in defending the proposed document on religious freedom, eventually promulgated as Dignitatis humanae.

[9][10][11] Though thought to oppose any liberalization of the Church's policy on contraception, in 1966 he surprised his peers on the Pontifical Commission on Birth Control by saying that "it would not be too rash" to approve of artificial birth control as a deeper understanding of traditional teaching.