Education in the modern era expects institutions to help students to thrive. Not only may class engagement improve academic performance, but it may also enhance students' well-being. This study investigated class engagement and its motivation and academic achievements' effect on university students' mental well-being. 231 university students, with a mean age of 21.46, participated in this study. About 65% were senior students, and average GPA was 3.46 (SD = 0.32). For measurements, General Class Engagement Scale, Motivations for Class Engagement Scale, and Warwick-Edinburg Mental Well-being Short Form was employed. Academic achievement was measured by GPA. For data analysis, Pearson correlation analysis and regression analyses were conducted. Results show that class engagement, its motivations, academic achievement, and mental well-being were all positively correlated. Moreover, class engagement and its motivations positively predicted mental well-being. However, GPA was non-significant. Hence, in a group of mostly senior university students, class engagement was more significant than academic achievement regarding their mental well-being. Furthermore, about 91% of this study's participants were honor students (above 3 GPA). Therefore, being academically successful may not always be enough to be happy and well. However, educators may help students by emphasizing class engagement.