["Family Relations, Volume 75, Issue 2, Page 1084-1101, April 2026. ", "\nABSTRACT\n\nObjectives\nUsing four waves of data from the China Family Panel Study, this study examined the impact of household income trajectories on two key dimensions of adolescent mental well‐being (i.e., depressive symptoms and happiness) and explored the mediating mechanisms.\n\n\nBackground\nSubstantial evidence links socioeconomic status (SES) to adolescent mental health, yet few studies have examined how longitudinal SES trajectories shape mental well‐being and the mechanisms underlying these relationships.\n\n\nMethod\nUsing group‐based trajectory modeling, we analyzed a nationally representative sample of 3,491 Chinese families with adolescents aged 10 to 19, identifying five distinct income trajectories.\n\n\nResults\nAdolescents from stable lower middle‐income and decreasing‐income groups demonstrated more depressive symptoms compared with their counterparts in the stable high‐income group. These associations were primarily mediated by maternal mental well‐being and family material investment pathways. Additionally, adolescents in the stable low‐income group exhibited lower happiness levels than the stable high‐income group, with maternal mental well‐being partly mediating this association.\n\n\nConclusion\nBoth stable lower income and downward income mobility are significant predictors of poorer adolescent mental health outcomes, mediated through distinct pathways such as family stress and family material investment.\n\n\nImplications\nOur results underscore the critical role of SES trajectories in shaping developmental contexts that influence adolescent mental well‐being.\n\n"]