By 1630 the Le Nain brothers had established a common workshop in Paris. They remained unmarried and are traditionally said to have worked in harmony, often collaborating on the same picture. In 1648 all were received into the newly founded French Academy. The “Le Nain problem” of determining which of them painted what is complicated because no signed work bears a first initial and no work completed after 1648 is dated. Evaluation of the three personalities early in the 20th century was therefore based on what was traditionally known of each brother and on the dubious establishment of three stylistic groups. Art scholars today no longer try to attribute individual works, and the three brothers are treated as a single artist. Their portraits of peasants and beggars remain their most important works, although A Blacksmith in His Forge was one of the most-admired and most-copied paintings in the Louvre in the 19th century. Their domestic scenes of peasant life depict humble people with human dignity, with a classical composure that is characteristically French.

Mathieu became an official painter to the city of Paris (1633) and was made a chevalier. He excelled in large compositions and in portraiture. His career was prosperous, and, from the large number of portraits and religious works produced in his studio, he must have had several assistants.