The effectiveness of a mindset intervention in secondary vocational education and training was investigated. Students (n = 55) enrolled in a mathematics course participated in an online mindset intervention for a period of 10 weeks. Both pretest and posttest scores on mindset and mathematics results were compared with students who did not get a mindset intervention. We expected that students in the intervention group will show a growth in mindset and therefore have better results on the mathematics test at the end of the course compared to students in the control group. This effect will be stronger for lower performing students and students with a fixed mindset. Results showed that at the end of this period, the scores on the mathematics test were higher for both groups, but that there were no significant differences between groups. Furthermore, there was no effect of the intervention on the mindset of the students and there was no significant relation between mindset and scores on mathematics. Overall, our findings are in line with an increasing body of studies that did not find a relation between mindset and academic achievement.