Research on the link between compassion toward others (CTO) and subjective well-being (SWB) has gained increasing attention in recent years. However, studies about the prospective relationship between these two constructs, as well as its underlying psychological process (i.e., mediators), are limited. For this reason, we conducted long-term three-wave longitudinal research, with six months between waves, among a large and representative sample of Chileans (N = 1477) to analyze the longitudinal relationship between CTO and SWB. First, we hypothesized that CTO longitudinally predicts higher SWB (H1). Second, we theorized that beneficence, a sense of having a prosocial impact on others, would mediate the CTO–SWB link (H2). To test the hypotheses, cross-lagged panel models (CLPMs) were conducted. Our hypotheses were supported by the data. The results of Model 1 show that CTO prospectively predicts higher SWB, thus confirming the first hypothesis. Model 2 shows that CTO prospectively predicts higher SWB indirectly through the mediation of beneficence, which supports our second hypothesis. The implications of the findings of this study and suggestions for future research are discussed from an evolutionary perspective.