<p>Childhood experiences play a crucial role in shaping lifelong happiness, yet their long-term implications remain underexplored. Drawing on life course and multidimensional capital perspectives, we examine the association between childhood left-behind experiences (LBE) and adult happiness using panel data from the China Family Panel Studies (CFPS). The analysis is based on 4,464 Chinese individuals (mean of age = 30.5, with 48.3% being male) with LBE measured between the ages of 0 and 12. We find that LBE is associated with lower adult happiness. Mediation analysis indicates that the negative association operates primarily through declines in traditional human capital—especially physical and mental health—and weakened family social capital, including relationships with parents and spousal intimacy. In contrast, the role of new human capital is limited. Moreover, a robust kinship network may partially mitigate the adverse consequences associated with LBE. We further use the KHB method to decompose the association and quantify indirect pathways. Overall, the findings are consistent with the long-term implications of early adversity and underscore the relevance of considering early interventions to support left-behind children (LBC) and promote better life outcomes.</p>