Mindfulness-based interventions (MBIs) for engineering design are promising for their stress management, cognition and well-being benefits. Prior work concluded that engineering design is stressful and that each engineering design stage has unique stressors. This non-randomized study investigated the effect of an MBI on students' cognitive stress and final design creativity during a multistage, hands-on design assessment. Data were collected using surveys, project deliverables and follow-up interviews. While no significant increase was found in students' measured state mindfulness due to the intervention, students in the MBI condition were more likely to perceive the intervention positively compared to students in the control condition (alternative use tasks). Students in both conditions were found to have similar levels of state stress, which indicates that the MBI had no observable effect on students' measured stress during design. Although students in the MBI condition were found to produce higher-quality final designs, there were no differences in design creativity or novelty. When data were clustered to identify types of student experiences, state mindfulness was found to meaningfully contribute, but state stress was not. Future research should continue to investigate MBIs in engineering design as a potential approach to improve design education and outcomes.