This study examined the effects of mental imagery ability (MIA) on the efficacy of two positive psychology interventions (PPIs) to enhance well-being. Participants (N = 210) were randomly assigned to either: Three Good Things (TGT), Best Possible Selves (BPS), or a control group and completed well-being questionnaires pre and post intervention. ANCOVA results partially supported the hypothesis that the interventions would significantly increase well-being (measured by the WEMWBS, PA and NA) compared to the control group. Correlations partially supported the prediction that greater effort and motivation towards the PPI would relate to greater increases in well-being. MIA was not found to influence the efficacy of the PPIs, hence, refuting the final hypothesis that participants with high MIA would report greater post-intervention increases in well-being than participants with low MIA (measured by imagery vividness and controllability scales). Well-being was positively correlated with MIA suggesting that improving MIA might facilitate an increase in well-being regardless of PPI use.