Rumination, characterized by an individual’s persistent and intense reflection on negative experiences, significantly impacts mental well-being. The main objective of the present study was to examine the associations between rumination (reflection and brooding) and mental well-being, through a serial mediation analysis that integrates loneliness and task-oriented coping as intervening variables. The research involved 750 university students, comprising 552 females (73.6%) and 198 males (26.4%). Self-report data were collected using valid and reliable instruments assessing rumination, loneliness, task-oriented coping, and mental well-being. The findings of the study indicate that loneliness and task-oriented coping have a serial mediating role in the relationship between rumination (reflection and brooding) and mental well-being. The results demonstrated that both reflection rumination and brooding rumination increased loneliness and increased loneliness influenced mental well-being by decreasing task-oriented coping behaviors. These results underscore the detrimental influence of rumination on the mental well-being of university students. The findings of the study were discussed and interpreted in detail in the light of the relevant literature.