Background: Research suggests growth mindset interventions can support student achievement, particularly among students at risk of academic struggle, but it remains underspecified which at-risk populations will benefit from such interventions. Objectives: This study aimed to experimentally evaluate whether highly (vs. moderately or minimally) nontraditional community college students would benefit from a growth mindset intervention and whether their performance differed at baseline and after the intervention. Method: A sample of 155 students in introductory psychology at a 2-year community college was randomly assigned to complete a growth mindset or control intervention, and all participants completed an extensive background survey. Results: Results showed improved exam performance among highly nontraditional students in the mindset (vs. control) condition but no benefit among other students. Highly nontraditional students showed similar pre-intervention performance to peers, but their exam scores declined more over the semester than did control peers. Conclusion: Growth mindset interventions may offer a low-cost strategy for supporting nontraditional community college students at risk of a downward performance trajectory. Teaching Implications: Instructors can use easily administered mindset interventions to support at-risk students' academic performance. They may also wish to evaluate the performance-related messages they convey to students and work to create a growth-oriented classroom environment.