Kahr grew up in New York City, where he attended the Fieldston School of the Ethical Culture Society. He went to Harvard University in 1957 and graduated three years later, in 1960. He earned his Ph.D. in mathematics in 1962 at the Massachusetts Institute of Technology. His thesis was "A Minimal Reduction Class for the Entscheidungsproblem". Following that he attended Harvard Business School. For his role in forming the credit card business, he was interviewed by the PBS Frontline documentary "Secret History of the Credit Card".