Ecumenism played a vital role in the life of pastor Boegner, in particular the formation of the World Council of Churches in 1938 (which first met in Amsterdam in 1948), in which Boegner was to hold a variety of functions. Apart from this predominantly Anglican-Protestant ecumenical engagement, Boegner was also active on the Catholic-Protestant front (in which context he attended the third and fourth sessions of Vatican II in 1964-1965) and came to discover the Taizé community, for which he developed a certain fascination. This article highlights the personal and social traits of Boegner that facilitated this noteworthy French Protestant’s international ecumenical career. Thereafter, it analyses his ecumenical position, founded on his view on the unity of the Church beyond the diversity of churches, on interconfessional spirituality, and Christian witness in the society. As a missionary of ecumenism in France and the world, Boegner, who had served as president to the Fédération Protestante de France (1929-1961), was to give expression to his disappointment at what in his eyes constituted an ecumenical timidity on the part of French Protestants.