Introduction <p>While it is common for medical students to begin medical school directly following a post-secondary degree, a small portion enter training after having established a professional career in a different field. Despite considerable research on professional identity formation in medical education, little is known about how prior professional identities influence socialization into the medical profession.</p> Methods <p>This study employed a qualitative descriptive analytic approach to explore the experiences of medical students at McMaster University (Hamilton, Canada) who had previous professional careers. Drawing on the <i>Theory of Identity Customization</i>, semi-structured interviews prompted reflection on career trajectories, curricular experiences, and how tensions between previous and new professional identities were navigated.</p> Results <p>Thirteen medical students with prior careers in healthcare, academia, business, and other fields participated. These students all pointed to poignant experiences of identity violation during training, and described reconciling these violations through processes of identity enriching, patching, splinting, and toggling. They emphasized that their previous identities were simultaneously isolating but also a source of resilience. Tensions related to the hierarchical structure of medical education were particularly salient for these students, most notably during the transition from pre-clerkship to clerkship, as many struggled with the shift from positions of presumed competency in previous fields to learner positions on medical teams.</p> Discussion <p>This study reveals the complex identity negotiation processes that medical students with prior careers undergo during training. There is a need for additional scholarship, which can guide medical schools in providing tailored support systems and curricula that address the unique needs of these students.</p>