During his tenure, he expanded the scope of the collection to include abstract expressionist painting as well as sculpture, established the long-term loans program in 1953 and the Guggenheim International Award in 1956.
He was also involved in the final years of the construction of the Frank Lloyd Wright designed museum building, during which time he had an antagonistic relationship with the architect.
[2] Sweeney collected works by: Alexander Archipenko, Constantin Brâncuși, Alberto Burri, Alexander Calder, César, Alberto Giacometti, Lucio Fontana, David Hayes, Willem de Kooning, Fernand Léger, Joan Miró, Piet Mondrian, Pablo Picasso, Jackson Pollock and Pierre Soulages.
[3] In the late 1960s, Sweeney was a consultant to the National Gallery of Australia during its establishment, advising on issues concerning the display and storage of art.
According to the Gallery's first director, James Mollison, "the size and form of the building had been determined between Colin Madigan and J.J. Sweeney, and the National Capital Development Commission.