The correlations between a low family socioeconomic status (SES) and adolescents’ poor academic outcomes have been widely documented. However, the mechanisms through which family SES is associated with adolescents’ academic achievement are not well understood. Therefore, this study examined the mediating roles of different types of parental involvement (i.e., parental school-based involvement, academic socialization, and home-based involvement) and the moderating role of adolescents’ subjective social mobility in the relationships between family SES and academic achievement by using multisource data. The valid sample consisted of 842 adolescents (${\overline{X}}_{\mathrm{age}}$ = 12.28 years, 40.97% girls) who were recruited from five township public schools in China. The results indicated that all three types of parental involvement partially mediated the relationships between family SES and adolescents’ Chinese and math achievement; parental school-based involvement had the strongest mediating effects, followed by academic socialization, and home-based involvement had the lowest mediating effects. Moreover, the path from parental home-based involvement to Chinese achievement in the mediation model was moderated by the adolescents’ subjective social mobility. In conclusion, parental involvement is an important mediating mechanism through which a low family SES impedes adolescents’ academic achievement, and adolescents’ subjective social mobility can play a buffering role in the relationship between parental home-based involvement and Chinese achievement.