As the adult learner population in higher education continues to increase, there has been a renewed focus on how to best serve this student population in recent years. While adult learners are more likely to take online classes and to enroll part-time, engagement and retention efforts have not heavily focused on part-time online adult learners, particularly when it comes to how best to engage these learners virtually. Utilizing retrospective data from an adult learner corporate partnership program at a four-year public research university, this study examines the effectiveness of online peer mentoring on course completion among part-time adult learners in an online precollege program. Part-time adult learners who received online peer mentoring in a precollege program for corporate partnership students were compared to part-time adult learners who did not receive online peer mentoring in a precollege program for corporate partnership students. Chi-square analyses were performed to determine the association between online peer mentoring and course completion for part-time adult learners overall as well as for part-time adult learners of color and male and female part-time adult learners. Results indicate that there is a statistically significant association between online peer mentoring for part-time adult learners, part-time adult learners of color, and male part-time adult learners. Based on the findings of this study, it is recommended that institutions consider ways to further engage part-time online adult learners to better meet and support their specific learning needs. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]