Although population density and phenotype are known to alter the mating strategies of individuals, their impacts on reproductive success have not been fully investigated. We examined genetic mating system variation (number of mating partners), reproductive success (offspring number) and sperm competition intensity (sires per litter) in semi-natural populations of Brandt’s voles (Lasiopodomys brandtii) under low-, medium- and high-densities. We used 12 microsatellite genetic markers and parentage assignment of 3,516 voles, with the aim of identifying the number of mating partners and offspring produced by founder individuals. We provided strong evidence that individuals with more mating partners experienced increased reproductive success, as measured by the production of more offspring, compared with individuals that had fewer mating partners. Further, we found that large males had more mating partners (especially in high-population density enclosures) and also produced more offspring relative to smaller males. A high density of founder individuals led to more sires per litter but greater paternity share across litters (reduced paternity skew). Our results suggest that a polygamous mating system and increased body size should be favored by natural selection, and density-dependent variation in mating strategy would be sufficient to stabilize reproductive fitness under unfavorable conditions such as high founding density.