Armstrong was born in Winchester, Massachusetts. She enrolled at Hampshire College in Amherst, Massachusetts intending to be an artist, but transitioned to art history and cultural studies, earning a BA in American Studies in 1974. After graduating, she worked as a grants administrator at the National Endowment for the Humanities (1976–9) before entering graduate school at the University of California Berkeley to earn an MA in the History of Art in 1982. While at Berkeley, she worked as a research assistant for the San Francisco Museum of Modern Art and as a curatorial assistant for the Lowie Museum of Anthropology, where she contributed to the 1983 publication “The Anthropology of World’s Fairs: San Francisco's Panama Pacific International Exposition of 1915." An essay based on the subject of her master's thesis, "American Scene as Satire, the Art of Paul Cadmus in the 1930s," appeared in "Arts Magazine" in March 1982.