Abstract Married individuals have substantially lower risks of morbidity and mortality and enjoy better health than unmarried people. Yet marriage may represent only one stage over an individual’s life course, and health benefits of marriage may depend on marital age and duration. Furthermore, cohabitation before marriage and union status after marriage may also contribute to health outcomes later in life. Recent work has moved from research on health disparities by marital status or transition to focus its attention on union history and health. However, measures of union history do not capture cumulative effects that unfold over the life course and neglect how earlier union experiences may shape the health consequences of later unions. In this application, we propose a life course perspective and posit that union history (including timing, duration, and sequencing of singlehood, marriage, cohabitation, dissolution, and/or widowhood) in earlier adult life influences health outcomes in midlife. The data from the 2016 wave of the National Longitudinal Survey of Youth 1979 (NLSY79) represent a milestone in which all respondents had reached age 50 and answered health questions designed for that age. We apply sequence analysis to capture the complete structure of union history. Our approach is innovative in that we identify prototypical union history trajectories through measures of timing, duration, and sequencing of individuals’ union events in earlier adult life, which correspond well with how life course experiences may have cumulative effects on midlife health. We investigate how gender, race/ethnicity, and social class shape union history trajectory, how union history trajectory influences midlife health outcomes, and how the relationships between union history trajectory and midlife health outcomes vary by gender, race/ethnicity, and social class. This work will yield knowledge of how increasingly complex union history trajectories contribute to growing health disparities among America’s diverse populations.