He served as curator at the Getty Research Center before moving to Pennsylvania where he was senior curator at the Palmer Museum of Art. In addition to his publications Willumson has organized exhibitions on American daguerreotypes, the artists of the Stieglitz circle, the Pincus collection of contemporary art, the video work of Bill Viola, and the photographs of Allan Sekula. He has held affiliate and visiting faculty positions at the University of California, Irvine, and at The Pennsylvania State University. His forthcoming book will be published by the University of California Press and is entitled Iron Muse: Photographing the Transcontinental Railroad.