Sherwin was born on July 2, 1937, in Brooklyn, New York, to Mimi (nee Karp) and Harold Sherwin. His mother was a homemaker who also worked administrative jobs while his father was a children's clothing manufacturer. He graduated from James Madison High School in Brooklyn after which he enrolled in Dartmouth College aiming to pursue medicine. However, he went on to study geology and philosophy, eventually graduating in 1959 with a Bachelor of Arts degree in history. Sherwin earned his PhD in history at the University of California, Los Angeles. His doctoral thesis, studying Harry S. Truman's atomic strategy, became his first book, A World Destroyed.