A legacy at the National Gallery of Art in Washington, D.C., Walker worked with the MFAA following World War II as a special advisor to the Roberts Commission. Due to sustaining disabilities from contracting polio as a young boy, he was not capable of serving in the military, but went to Europe in 1945 to offer his assistance. From July to August 1945 Walker was the representative of the American Commission Abroad. While in Europe, he inspected repositories and collecting points in Germany and Austria and later reported his thoughts on the achievements of the MFAA at a Commission Meeting. Walker was, however, in support of the infamous removal of 202 German-owned works of art to the United States.

Walker graduated summa cum laude from Harvard University in 1930. While at Harvard, he took the famed Paul Sachs ‘Museum Course’ and further developed his interest in the arts. In 1928, he co-founded the Harvard Society for Contemporary Art with Lincoln Kirstein, also a future Monuments Man. Walker continued his studies at Villa I Tatti in Florence with Italian Renaissance scholar Bernard Berenson for the next three years. In 1935 he was named professor in charge of fine arts at the American Academy in Rome. While in Rome he met and married Margaret Drummond, the eldest daughter of the British ambassador and the 16th Earl of Perth. During this time Walker also aided in negotiations with Berernson to bequeath the Villa I Tatti to Harvard upon his death for use as a Center for Renaissance Studies.

A childhood friend of Paul Mellon (whose father, Andrew Mellon was the founding force behind the creation of the National Gallery of Art in Washington), Walker learned of the art patron’s plans to build a national museum. He wrote to Mellon and was soon appointed a position at the National Gallery of Art. In 1938, Walker arrived in Washington to supervise construction after the sudden deaths of both Mellon and the principal architect, John Russel Pope. He was named the first chief curator of the museum under director David Finley in January, 1939 and worked tirelessly to install pieces acquired from the Mellon estate as others. In 1956, Walker succeeded Finley as director, and remained in the position until 1969. Under his directorship, he continued to build relationships with the Mellon family as well as new donors, and acquired works such as Leonardo da Vinci’s Ginevra de’ Benci and Rembrandt’s Aristotle with the Bust of Homer. Walker also used his friendship with Jacqueline Kennedy to secure the 1963 loan of the Mona Lisa to the National Gallery, an exhibition that was without precedent.

As an art director with an extensive art history background, Walker wrote monographs on artists including James McNeill Whistler and John Constable, and was of the belief, like his mentor Berenson, that an art museum’s true strength was drawn from the quality of its collection. He consistently used his connections to draw support for the National Gallery of Art through new donors, and wrote in his autobiography Self Portrait with Donors that “it is axiomatic that the undertaker and the museum director arrive almost simultaneously,” a statement which his career truly exemplified.