Reproductive success is often difficult to ascertain in reptiles because of inconspicuous reproductive behaviors such as courtship that occurs in aquatic habitats and cryptic breeding outcomes such as multiple paternity and sperm storage. We corroborated field observations of mating behavior with genetic parentage analysis to assess the mating system and reproductive success in the rare Blanding's Turtle (Emydoidea blandingii) within a fragmented landscape in Will County, Illinois. During four consecutive years of radio-telemetry monitoring, we observed promiscuous mating behavior among males and females. Parentage was strongly skewed (one male sired 38% of 272 offspring) and multiple paternity was rare (11% of 28 clutches). Both males and females produced offspring with multiple individuals over the course of the study, but parentage did not always correspond with observed mating attempts; only 41% of male mating attempts observed in the field corresponded with sired offspring. For males, number of female mates inferred from paternity analysis was positively correlated with total number of offspring sired. No correlation was found between relatedness of observed mating pairs and hatching success. Repeat paternities in clutches were detected among years, with one confirmed instance of across-season sperm storage. High variation in reproductive success and low levels of multiple paternity in Blanding's Turtle in Will County compared to other areas may be attributed to habitat modification, small population size, and disruption of the mating system.