Objective <p>Middle-aged and older cancer survivors, who face both age-related and cancer-related stressors, are particularly vulnerable in terms of mental functioning. From a life-course perspective, this study investigates the long-term associations between family-internal adverse childhood experiences (ACEs)—specifically childhood parental mental distress—and family-external ACEs—childhood peer bullying and friendship—and mental functioning in this population.</p> Methods <p>Data were drawn from 419 cancer survivors (mean age = 63.41, SD = 9.64) and 8209 non-cancer respondents (mean age = 59.64, SD = 8.67) from the 2014 and 2020 waves of the China Health and Retirement Longitudinal Study, a nationally representative survey employing a stratified multi-stage cluster sampling design. Structural equation modeling was used to test the hypotheses, following STROBE reporting guidelines.</p> Results <p>Childhood parental mental distress was significantly and negatively associated with mental functioning in cancer survivors, with childhood peer friendship and bullying mediating this relationship. Among the mediators, the association between peer friendship and mental functioning was stronger than that between peer bullying and mental functioning. Furthermore, these associations were more pronounced in cancer survivors than in non-cancer respondents.</p> Conclusions <p>Both family-internal and -external ACEs are significantly associated with lower levels of mental functioning in cancer survivors, with family-external ACEs serving as mediators.</p> Implications for Cancer Survivors <p>Given that the mental functioning of middle-aged and older cancer survivors is undermined by the combined effects of ACEs-related, age-related, and cancer-related stressors, intervention strategies should adopt an integrative life-course perspective that concurrently addresses all three domains to effectively protect their mental functioning.</p>