Increased relatedness and changes in pollination services in small plant populations can alter their mating system. Often, these alterations involve increased rates of selfing and/or biparental inbreeding. Such changes can have critical consequences for population viability; via inbreeding depression, increased levels of inbreeding can result in lower population growth rates and increased risks of local extinction. Therefore, understanding how the mating system varies with natural population size represents a critical step toward understanding the factors that affect population persistence. In this study, we examine how the mating system, characterized by the outcrossing rate, biparental inbreeding rate, effective number of male parents (Nep), and the inbreeding coefficient varies with population size and density across eight natural populations of the self-compatible biennial Sabatia angularis (Gentianaceae). We further explore how variation in population size might lead to changes in genetic diversity across these populations, which can also affect average population fitness. Our results reveal a significant, positive relationship between population size and the outcrossing rate, demonstrating that small populations of this species have higher selfing rates. Neither population size nor density, however, accounted for significant variation in any of the other mating system or genetic diversity parameters evaluated. We also provide evidence from a related study examining inbreeding depression in natural populations of S. angularis that lends support to the notion that increased levels of selfing in small populations of this species could have important consequences for population persistence.