The global mental health crisis is a pressing concern, with college students particularly vulnerable to rising mental health disorders. The widespread use of smartphones among young adults, while offering numerous benefits, has also been linked to negative outcomes such as addiction and regret, which can significantly impact well-being. Leveraging the longest longitudinal dataset collected over four college years through passive mobile sensing, this study is the first to examine the relationship between college students’ smartphone behaviors and their mental health in real-world settings. Our findings provide key insights into variations in smartphone usage across genders and locations, offering a deeper understanding of the interplay between digital behaviors and mental health. We also highlight future directions for research aimed at mitigating adverse effects and promoting digital well-being in this population.