How does the United States maintain the highest-quality research and teaching in its professional science workforce and ensure that those in this workforce are effectively trained and representative of national demographics? In the pathway to science careers, the postdoctoral stage is formative, providing the experiences that define the independent work of one's first faculty position. It is also a stage in which underrepresented minorities (URMs) disproportionately lose interest in pursuing academic careers in science and, models suggest, a point at which interventions to increase proportions of URMs in such careers could be most effective. We present a mixed-methods, case study analysis from 17 years of the Fellowships in Research and Science Teaching (FIRST) postdoctoral program, to our knowledge the largest and longest continuously running science postdoctoral program in the United States. We demonstrate that FIRST fellows, in sharp contrast to postdocs overall, are inclusive of URMs (50% African American; 70% women) and as or more successful in their fellowships and beyond as a comparison group (measured by publication rate, attainment of employment in academic science careers, and eventual research grant support). Analysis of alumni surveys and focus group discussions reveals that FIRST fellows place highest value on the cohort-driven community and the developmental teaching and research training the program provides.