Purpose The Professional Identity Essay (PIE) is a theory and evidence-based Medical Professional Identity Formation (MPIF) measure. We describe trajectories of PIE-measured MPIF over a 4-year US medical school curriculum. Methods Students write PIEs at medical school orientation, clinical clerkships orientation, and post-advanced (near graduation) clerkship. A trained evaluator assigns an overall stage score to narrative responses to nine PIE prompts (inter-rater ICC 0.83, 95% CI [0.57 − 0.96], intra-rater ICC 0.85). Distribution of PIE stage scores across time points were analyzed in the aggregate and individual students were classified as Increase, Stable (no score change) or Decrease based on the trajectories of PIE stage scores over time. Results 202 students completed 592 PIEs from 2018-2023. There was a significant change in the proportion of PIEs in stages over time (X2 84.40, p < 0.001), 47% (n = 95) students were categorized in the Increase trajectory, 45.5% (n = 92) as Stable and 7.4% (n = 15) as Decrease. Older age and time-predicted stage scores change within trajectories (p < 0.05). Conclusions Medical students’ PIE stage scores increase over time with three distinctive trajectories. Further study is needed to explore the utility of this method for formative assessment, program evaluation, and MPIF research.