The development of critical thinking skills in recent college graduates is keenly requested by employers year after year. Moreover, improving these skills can help students to better question and analyze data. Consequently, we aimed to implement a training program that would add to the critical thinking skills of undergraduate students: Nebraska Science, Technology, Engineering, and Math 4U (NE STEM 4U). In this program, undergraduates provide outreach, mentoring, and science, technology, engineering, and mathematics (STEM) education to K-8 students. To determine the impacts of serving as an undergraduate mentor in this program on critical thinking, we compared undergraduate mentors (intervention group) with nonmentor STEM majors (nonintervention, matched group) using the valid and reliable California Critical Thinking Skills Test (CCTST) as a pre/post measurement. Importantly, before the intervention, both NE STEM 4U mentors and nonmentor undergraduates scored similarly overall on the CCTST. However, the posttest, carried out one academic year later, indicated significant gains in critical thinking by the NE STEM 4U mentors compared with the nonmentors. Specifically, the math-related skills of analysis, inference, and numeracy improved significantly in mentors compared with nonmentors.