Pflanze was the son of Otto Paul Pflanze (1885-1944) and Katrine Mills Pflanze (1886-1959). He attended Maryville College and Yale University before his graduate education at Yale was interrupted by World War II, where he served in the Pacific Theater in the Army Air Corps as an intelligence officer documenting the war effort. After the end of the war, Pflanze worked for the State Department and was in the Berlin Airlift accompanying the delivery of supplies in 1948, before finally resuming his graduate studies at Yale. He received his PhD in 1950, and taught at the following institutions (listed in chronological order): New York University, University of Massachusetts at Amherst, University of Illinois, University of Minnesota, Indiana University, and Bard College. From 1970 to 1971, Pflanze was a Scholar in Residence at the Institute for Advanced Study in Princeton, New Jersey. From 1976 to 1985, Pflanze was the Editor of the American Historical Review, headquartered in Bloomington, Indiana.