In 1928 he became the first Director-General of the Palais des Beaux-Arts, Brussels, creating La Maison d’Art in December 1933; cofounded the Revue Internationale de Musique (1936–1952).
During the First World War Leirens worked for the Belgian services in London and in 1919 became the secretary of Fondation Universitaire, after which he abandoned musical composition.
[5] Over his career subjects were to include Jacob Epstein (1929), Roger Fry (1933), August Vermeylen (1934), Andre Gide (1935), Colette (1935 and 1950), François Mauriac (1935, 1957),[6] James Ensor (1933), Paul Valéry (1934),[7] Osip Zadkine (1935), Aristide Maillol (1935), Béla Bartók (1944),[8] Henry Moore (1946), Marc Chagall (1948, 1952), Gaby Casadesus (1950),[9] Jean Cocteau (1957), Charles Leplae (1958), Paul Delvaux (1958), Andre Malraux (1958), Gaston Bachelard (1958),[10] Eugène Ionesco (1958), Franz Hellens (1960), René Magritte (1960)[11][12] With the outbreak of the Second World War, in 1940 Leirens was invited by the New School for Social Research, New York, to give courses in photography and musicology.
[19] Returning to Belgium in 1952 Leirens married Virginia Haggard (McNeill) the daughter of a British diplomat, and previously the wife of painter John McNeill, whom Leirens met with Marc Chagall with whom she had lived for seven years,[20] and mother of their son, the actor David McNeil (born 22 June 1946); within three months Chagall had married his housekeeper, Valentina Brodsky[21] and the six-year old David went to live with his mother and step-father.
During this time Leirens resided in Paris while also working in Belgium and made an extensive series of portraits of writers, artists and musicians for the Archives of the Ministry of Culture.
The following year he returned full-time to live in Brussels where he continued his concerts for the Maison d'Art as well as his photographic work, and mentoring his compatriot, the young Yves Auquier (1934-).