From 1908 to 1910 he worked as an assistant to Seth Eugene Meek at the Field Museum of Natural History in Chicago.
From 1919 to 1925 he worked as an ichthyologist at the U.S. Bureau of Fisheries in Washington, D.C. From 1920 to 1924 he was a consultant and investigator with the United States Public Health Service.
In 1949 the United States Department of the Interior posthumously honored him with its Distinguished Service Award.
Hildebrand's research focused on the life of turtles, mosquito control and the life of fish larvae, the early development of North American fish, studies on the Central American ichthyofauna, marine fishes in eastern North America, Panama and Peru, and revisions within the herring family.
Furthermore, Hildebrand was involved in the standard work Fishes of the Western North Atlantic.