I first show that Kuhn came to have doubts about physics soon after entering college but did not make up his mind to leave the discipline until 1947–1948 when a close association with Harvard’s President James B. Conant convinced him of the desirability of an alternative career in the history of science. I go on to maintain that it was realistic for Kuhn to prepare for such a career in essentially autodidactic ways both because he enjoyed Conant’s patronage and because he could expect that his credentials in physics would be an asset in this relatively young interdisciplinary specialty. I then suggest that it was through his work as a teacher, researcher, and journeyman gatekeeper in the history of science that Kuhn gradually came to identify with the field. Finally, I argue that his training in physics, his teaching of general-education courses, and his hopes of influencing current philosophy of science helped shape his early practice as a historian of science. By way of epilogue, I briefly consider Kuhn’s path from his tenuring at Berkeley in 1958 to the appearance of The Structure of Scientific Revolutions in 1962.