The study investigated how clinical supervision affects the personal and professional development of first-time supervisees. The explanatory sequential mixed methods research design was employed in the study. Quantitative data was collected using a single group pretest-posttest experimental design to evaluate the impact of supervision on the personal and professional development of counselors in training. Individual interviews were conducted with 51 volunteer participants who were counselors in training at a state university in western Türkiye. Paired-sample t-tests were used to analyze the quantitative data, while content analysis was employed for the qualitative data. The findings showed that supervision significantly enhanced the counselors in training's effective counselor characteristics and self-awareness management strategies. However, there were no significant differences in hindering self-awareness and professional disposition. The qualitative analysis revealed that counselors in training who participated in supervision gained a deeper understanding of counseling procedures and techniques, increased trust in the profession, and improved self-efficacy. Counselors in training also highlighted the positive impact of their supervisor and group supervision format on their development. The results of the study suggest the significance of supervision in counselor education. It is recommended that the variables included in the current research and the effect of supervision should be examined in more studies.