Paternal age and past mating effort by males are often confounded, which can affect our understanding of a father's age effects. To our knowledge, only a few studies have standardized mating history when testing for effects of paternal age, and none has simultaneously disentangled how paternal age and mating history might jointly influence offspring traits. Here, we experimentally manipulated male mating history to tease apart its effects from those of paternal age on female fertility and offspring traits in the eastern mosquitofish (Gambusia holbrooki). Male age did not affect female fertility. However, males with greater past mating effort produced significantly larger broods. Paternal age and mating history interacted to affect sons' body size: sons sired by old-virgin males were larger than those sired by old-mated males, but this was not the case for younger fathers. Intriguingly, however, sons sired by old-virgin males tended to produce fewer sperms than those sired by old-mated males, indicating a potential trade-off in beneficial paternal effects. Finally, neither paternal age nor mating history affected daughter's fitness. Our results highlight that variation in offspring traits attributed to paternal age effect could partly arise due to a father's mating history, and not simply to his chronological age.