Research proposing that mindset interventions promote student achievement has been conducted at a frenetic pace nationwide in the United States, with many studies yielding mixed results. The present study explores the hypothesis that mindset interventions are beneficial for students only under specific circumstances. Using a randomized controlled trial with student-level random assignment within two public schools (School 1: n = 198 seventh-graders, 73% Black, 27% White, 53% male; School 2: n = 400 ninth-graders, 98% White, 2% Black, 52% male), this trial conceptually integrated elements from three evidence-based mindset interventions. It then examined two theoretically driven moderators of student performance following the transition to middle or high school: students’ racial backgrounds and students’ educational expectations. Results indicated that the intervention was effective for a particular subset of students—Black students with high educational expectations—resulting in higher grades over the course of the year. Among students with low educational expectations (regardless of race), the intervention did not impact grades. For White students with high educational expectations, the control activities actually benefitted grades more than the mindset intervention. Both theoretical and practical implications for mindset research are discussed.