Although he knew nothing about art at that point, he spent days haunting the corridors and galleries of the Louvre in an experience that was overwhelming revelation to him and that he later termed "heavenly disorder". He left the museum convinced he had found his calling. He successfully negotiated the sale of the lady's painting, and with the profit bought other pictures. Nathan took a gamble, totally abandoning his early career and putting all of his meagre income into acquiring works of art. He trained his eye by constant visits to museums, private collections, dealer's shops on both sides of the Seine, flea markets and the auction rooms of the Hôtel Drouot and the Galerie Georges Petit. Sheer will power led him to become a connoisseur in the fullest sense of the word. He was sought out for his expertise, which some collectors came to regard as nearly infallible.

In the late 1870s Nathan Wildenstein opened in Paris where he began buying and selling French paintings. Enamoured with the creative brilliance of the French ancien regime and the Enlightenment, Nathan was quick to see that the vagaries of fashion and taste had dispersed these underappreciated masterpieces into unjustified obscurity. He was endowed with an instinct for quality and an ability to recognise the masterpiece lost among a multitude of other objects. In 1890, he was able to move to the more prestigious location at 46, rue du Faubourg Saint-Honoré.

He and his wife were an ideal couple. Well-educated, an omnivorous reader, a talented hostess, Laure added a certain distinction to the marriage and was the soul of the family. A modern woman, she oversaw the firm's bookkeeping, the preparation of invoices and the correspondence exchanged between her husband and a growing number of clients.

A smart dresser, Nathan Wildenstein is said to have had a wry, jovial, entertaining and infectious sense of humour that endeared him to many with whom he came into contact. Moreover his enthusiasm about the works he was selling was absolutely contagious. By the turn of the 20th century, he was considered as one of the pre-eminent art dealers in Europe and with a flair for seizing opportunity the instant it appeared. Nathan Wildenstein's intrepid spirit meant that he had amassed a superb collection of Old Masters and established himself as one of the leading art dealers in Europe.