Hamilton was an authority on modern art and trained art historians and museum curators.
He wrote a number of important books on modern art, including Manet and His Critics and Painting and Sculpture in Europe 1880-1940, a survey that continues to be read by students. He was one of the first historians to recognize the importance of many modern artists, including Marcel Duchamp, with whom he maintained a long friendship.
As the director of the Sterling and Francine Clark Art Institute in Williamstown, he had a wide influence on art historians and curators.