Robert Plant Armstrong was born in Wheeling, West Virginia, on May 19, 1919, to Clarence Warren and Dorothy Johanna (Green) Armstrong. He served in the U.S. Navy from 1940-1943. Armstrong received a BA in English at the University of Arizona in1944, an MA from the Iowa Writers Workshop in 1946, and his Ph.D. in Anthropology at Northwestern University in 1957. He taught English at several colleges and universities, among them Montana State University, Bozeman(1946-1950), Balai Bahasa Inggeris,Jogjakarta, Indonesia (1955-1956), and University of Michigan, Ann Arbor in the summers of 1956-1957. He held several jobs in the book world, such as Traveler for the Houghton Mifflin Company (1945-1946), Field Editor for Harper & Bros., (1956-1958), Editor for Alfred A. Knopf (1958-1959), and was director of publications at the University of Arizona Press, Tuscon (1959-1960) before becoming Director of Northwestern University Press in 1960. In 1967 he was also appointed as a Professor or Arts and Sciences at Northwestern University. During these years, he accepted visiting appointments, including a Visiting Professorship of Art and Art history at State Univerisity of New York at Buffalo in the summer of 1970; Visiting Curator of African Art at the Buffalo Museum of Science, summer 1970; and Visiting Director of the University of Ibadan Press, 1972-1973.

Armstrong remained at Northwestern University Press for thirteen years, when he was fired in 1973 by the University administration while he was on leave at the University of Ibadan in Nigeria. The cause and justification for his firing are unclear, but in the collection there is a manuscript entitled "The Rise and Fall of Northwestern University Press" which discusses the problems which arise when the attempt is made to run a University Press on a business model. After leaving Northwestern he moved to the newly organized University of Texas at Dallas, serving in their Anthropology Department before moving to the College of Arts and Humanities, where he served until his death, of a brain tumor, in 1984. While at Dallas, he was instrumental in formulating their Aesthetics curriculum.

Armstrong was also a collector of African Art and possessed a fine collection. He combined all his interests (writing, anthropology, and African art) in the publication, in 1971, of his most important work, The Affecting Presence: An Essay in Humanistic Anthropology. He published more books, such as Forms and Processes of African Sculpture and Wellspring: On the Myth and the Source of Culture. He also published many articles. He loved words in all their forms, and besides acting as publisher as well as writing books and articles, he served as editor and consulting editor for several journals, such as Book Forum. He also delighted in repartee and in the collection there are examples of the sometimes off-colored limericks he wrote, as well as some serious poetry. Words used to describe Armstrong from the transcript of his memorial service are "intellectual loner," "aesthetic philosopher," "humanistic anthropologist," and someone whose work was characterized by "a strong rejection of western ethnocentrism."

Armstrong died after a long and debilitating battle with cancer on August 9, 1984. His collection of African art was auctioned to provide a scholarship for students at the University of Texas at Dallas.