In many species, male mating success is often influenced by both age and mass. Because mass often increases with age, it can be difficult to disentangle their separate contributions to older male mating success. In species in which age and mass vary independently, it may be possible to determine the influence of mass on male reproductive success. For example, declines in residual reproductive value with age often result in older males increasing their reproductive effort. Mass may have an important influence on this relationship, if having greater mass provides an older male with a larger resource pool to invest in reproductive effort. Here, I evaluated the effect of mass on the reproductive success of males in an insect species in which age and mass are uncorrelated, but mass is highly variable within age classes. Female thornbug treehoppers [Umbonia crassicornis (Amyot and Serville) (Hemiptera: Membracidae)] were given simultaneous choices of younger (22 d) and older (30 d) males under free-choice conditions. Both age and mass contributed to male mating success, but their effects were not additive. Older males had higher mating success than younger males, and among older males, heavier individuals obtained more matings. In contrast, mass did not influence younger male mating success. These findings suggest age and mass interact in their effects, and together provide heavier, older males with a significant mating advantage. Large mass may, therefore, represent an indicator of male courtship performance that is reliably revealed at older ages, which in turn may allow thornbug females to make more effective mating decisions.