A vast literature has documented the inverse association between educational attainment and U.S. adult mortality risk but given little attention to identifying the optimal functional form of the association. A theoretical explanation of the association hinges on our ability to describe it empirically. Using the 1979-1998 National Longitudinal Mortality Study for non-Hispanic white and black adults aged 25-100 years during the mortality follow-up period (N = 1,008,215), we evaluated 13 functional forms across race-gender-age subgroups to determine which form(s) best captured the association. Results revealed that the preferred functional form includes a linear decline in mortality risk from 0 to 11 years of education, followed by a step-change reduction in mortality risk upon attainment of a high school diploma, at which point mortality risk resumes a linear decline but with a steeper slope than that prior to a high school diploma. The findings provide important clues for theoretical development of explanatory mechanisms: an explanation for the selected functional form may require integrating a credentialist perspective to explain the step-change reduction in mortality risk upon attainment of a high school diploma, with a human capital perspective to explain the linear declines before and after a high school diploma.