OBJECTIVE To estimate the longitudinal bidirectional association between parent-child conflict and child externalizing and internalizing symptoms from the preschool years through adolescence. METHODS A nationally representative longitudinal study recruited 11,134 children at birth and followed them from December 2010 through June 2022. Primary caregivers completed validated measures at each follow-up, yielding data on parent-child conflict and child symptoms at ages 3, 5, 7, 9, and 13 years. Data were analyzed using a random-intercepts cross-lagged panel model. RESULTS Assessment was performed in children at ages 3 (n = 9,793), 5 (n = 9,001), 7 (n = 5,344), 9 (n = 8,032), and 13 years (n = 6,655); 50% of participants were girls. Externalizing symptoms at ages 3, 5, and 7 years were prospectively associated with parent-child conflict at ages 5, 7, and 9 years. The opposite associations were not observed, indicating a unidirectional link between child externalizing symptoms and later parent-child conflict during childhood. However, parent-child conflict at age 9 years was prospectively associated with child externalizing symptoms at age 13 years. The opposite prospective association was not observed, indicating a reversal in the direction of the association between externalizing symptoms and parent-child conflict during early adolescence. Prospective associations between parent-reported internalizing symptoms and parent-child conflict were not observed in either direction. CONCLUSIONS Findings underscore the importance of both the timing and the targeted nature of interventions for children's mental health. Supporting early self-regulation skills in children could be an effective intervention for preventing parent-child conflict later in development. CONCLUSIONS This study examined the relationship between parent-child conflict and children's mental health over time, using data from 10,172 families that participated in the Growing Up in Ireland project. Externalizing behaviors in children between the ages of 3 and 9 years, such as defiance, aggression, inattention, and impulsivity, predicted increased conflict with parents. Interestingly, this pattern reversed in children over the age of 9 years, with conflict in the parent-child relationship predicting externalizing problems in early adolescence. These findings emphasize the importance of child age in understanding family dynamics. They also suggest that helping young children develop self-regulation skills could reduce parent-child conflict, which in turn could benefit child mental health and family functioning. We worked to ensure sex and gender balance in the recruitment of human participants. We worked to ensure race, ethnic, and/or other types of diversity in the recruitment of human participants. We worked to ensure that the study questionnaires were prepared in an inclusive way. One or more of the authors of this paper self-identifies as a member of one or more historically underrepresented racial and/or ethnic groups in science. One or more of the authors of this paper self-identifies as a member of one or more historically underrepresented sexual and/or gender groups in science. One or more of the authors of this paper received support from a program designed to increase minority representation in science. We actively worked to promote sex and gender balance in our author group. We actively worked to promote inclusion of historically underrepresented racial and/or ethnic groups in science in our author group. While citing references scientifically relevant for this work, we also actively worked to promote sex and gender balance in our reference list. While citing references scientifically relevant for this work, we also actively worked to promote inclusion of historically underrepresented racial and/or ethnic groups in science in our reference list. The author list of this paper includes contributors from the location and/or community where the research was conducted who participated in the data collection, design, analysis, and/or interpretation of the work.