ImportanceBolstering the ranks of women and underrepresented groups in medicine (URM) among medical faculty can help address ongoing health care disparities and therefore constitutes a critical public health need. There are increasing proportions of URM faculty, but comparisons of these changes with shifts in regional populations are lacking.ObjectiveTo quantify the representation of women and URM and assess changes and variability in representation by individual US medical schools.Design, Setting, and ParticipantsThis retrospective cross-sectional study assessed US medical school faculty rosters for women and URM, including American Indian and Alaska Native, Black, Hispanic, and Native Hawaiian or other Pacific Islander faculty. US allopathic medical schools participating in the Association of American Medical Colleges (AAMC) Faculty Administrative Management Online User System from 1990 to 2019 (updated December 31 for each year), were included. Faculty data were analyzed from yearly cross-sections updated as of December 31 for each year from 1990 to 2019. For census data, decennial census data were used for years 1990, 2000, and 2010. Intercensal estimates were used for all other years from 1990 to 2019.Main Outcomes and MeasuresTrends and variability in representation quotient (RQ), defined as representation of a group within an institution’s faculty compared to its respective US county.ResultsThere were 121 AAMC member institutions (72 076 faculty) in 1990, which increased to 144 institutions (184 577 faculty) in 2019. The median RQ of women faculty increased from 0.42 (IQR, 0.37-0.46) to 0.80 (IQR, 0.74-0.89) (slope, +1.4% per year; P < .001). The median RQ of Black faculty increased from 0.10 (IQR, 0.06-0.22) to 0.22 (IQR, 0.14-0.41) (slope, +0.5% per year; P < .001), but remained low. In contrast, the median RQ of Hispanic faculty decreased from 0.44 (IQR, 0.19-1.22) to 0.34 (IQR, 0.23-0.62) (slope, −1.7% per year; P < .001) between 1990 and 2019. Absolute total change in RQ of URM showed an increase; however, the 30-year slope did not differ from zero (+0.1% per year; P = .052). Although RQ of women faculty increased for most institutions (127 [88.2%]), large variability in URM faculty trends were observed (57 institutions [39.6%] with increased RQ and 10 institutions [6.9%] with decreased RQ). Nearly one-quarter of institutions shifted from the top to bottom 50th percentile institutional ranking by URM RQ with county vs national comparisons.Conclusions and RelevanceThe findings of this cross-sectional study suggest that representation of women in academic medicine improved with time, while URM overall experienced only modest increases with wide variability across institutions. Among URM, the Hispanic population has lost representational ground. County-based population comparisons provide new insights into institutional variation in representation among medical school faculty.