Ailsa Mellon Bruce developed her interest in art collecting following the example of her father, Andrew W. Mellon, who founded the National Gallery of Art. After her father's death in 1937, she became deeply devoted to protecting and advancing his institutional legacy. During the directorship of John Walker from 1956 to 1969, she emerged as the museum's single greatest benefactor, providing millions of dollars for purchases regardless of her personal enthusiasm for specific works, so long as the director assured her they were needed. In 1968, she and her brother, Paul Mellon, donated twenty million dollars to build an annex to the museum. Upon her death in August 1969, she bequeathed the majority of her fortune to the Avalon Foundation, which later merged with her brother's Old Dominion Foundation to become the Andrew W. Mellon Foundation. She also left all her paintings, drawings, and watercolors, aside from family portraits and busts, to the National Gallery of Art.