He is most famous for his survey of collector’s marks on drawings and prints, which is still a reference work for specialists today and continues to be enriched in the form of a growing online database.

Inspired by the example of the private American foundations he had visited while living in the United States during the Second World War, he decided in 1947 to create the Fondation Custodia (whose name means "safekeeping" in Latin) and to transfer his collection and his assets there.

The Fondation Custodia continues to acquire works to enrich the Lugt Collection and regularly organises exhibitions and guided tours of the Hôtel Turgot.

[5] The collection, continually added to that formed by Lugt, includes works by Dutch, Flemish, Italian, French, Danish, British, German artists, and features over 7,000 Old Master drawings, 15,000 prints, and 450 paintings.

Other schools and painters from later centuries are represented by works from Sofonisba Anguissola, Paolo Porpora, Jacques Linard, Nicolas de Largillière and Francesco Guardi.

Among the famous artists, the foundation holds letters from Michelangelo, Albrecht Dürer, Rembrandt, Jean-Georges Wille, Paul Gauguin, Henri Matisse and Dimitri Bouchène.