This study explored whether adolescents’ introspection increased while engaged in a mindfulness-based intervention (MBI). Using a quasi-experimental design, the researchers identified high school students (n = 13) with elevated levels of anxiety to participate in an 11-week, multiphase MBI. Participants’ introspection ability was assessed by comparing self-reported levels and observed percentage of time calm as recorded by a neurofeedback (NFB) device three times a week during the program. Using general linear modeling, results indicated a significant interaction effect with moderate effect sizes between self-reported scores by phase on NFB scores, F(2, 407)=4.12, p = 0.017 and a significant interaction between sex and self-reported scores on NFB scores overall, F(1, 407)=5.05, p = 0.025. Female participants appeared to have increased introspection compared to males. Results support the use of MBIs to foster introspection in adolescents. The MBI with an NFB device appeared to improve introspection, a key factor in accurate self-report.