Research has suggested that mindfulness can help combat stress, increase self-compassion, and improve well-being. Although mindfulness is also beneficial for student-athletes, little is known about how developmental differences due to age may influence intervention efficacy. The present study investigated whether 6-week Mindful Sport Performance Enhancement (MSPE) training had a differential effect on younger (first and second year) compared to older (third and fourth year) college student-athletes’ mindfulness, sport anxiety, emotion dysregulation, and satisfaction with both attention/emotion regulation and physical sport performance. Participants were 39 younger and 21 older female athletes from three teams (lacrosse, field hockey, track & field) at a private university in the United States, ranging in age from 18-22. Multiple 2 × 2 mixed-design ANOVAs revealed significant reductions in sport anxiety as well as significant improvements on self-ratings of satisfaction with both attention/emotion regulation and physical sport performance. Older student-athletes improved more than their younger teammates on sport mindfulness, and independent samples t-tests indicated that older student-athletes reported more frequent mindfulness practice during workouts, team practice, and competitions. Linear regressions found that age predicted reductions in sport anxiety and improvements in dispositional mindful acceptance. Finally, student-athletes perceived MSPE to be moderately to highly successful in promoting change in a variety of performance-related areas such as anxiety reduction, focus, and the ability to let things go. The implications of these results are presented, as well as suggestions for future research on mindfulness mental training with college athletes.