Objective: Fantasy realization theory (Oettingen, 2012) proposes that fantasizing about a desired future or dwelling upon negative reality rarely changes behavior whereas mentally contrasting fantasy with reality can be an effective behavior change technique. This is because mental contrasting energizes people to overcome obstacles that stand in the way of their desired future. The present study tested whether mental contrasting promotes rates of physical activity among overweight, middle-aged, and low-SES men. Method: A randomized controlled trial was conducted with members of an angling club in the north of England (N = 467). At baseline, participants completed a postal questionnaire that measured cognitions about physical activity. The intervention was embedded in the questionnaire for relevant participants. Behavior was followed up via telephone at 1 month and 7 months postbaseline. The key outcome measure was a validated, self-report measure of physical activity (Godin, Jobin & Bouillon, 1986) taken at all three time-points. Results: Longitudinal, explanatory, and intention-to-treat analyses each indicated that mental contrasting was effective in enhancing rates of physical activity. Mental contrasting also aided the translation of beliefs about the value and worth of physical activity (instrumental attitudes) into action. Conclusion: Mental contrasting appears to be an effective self-regulatory intervention for promoting physical activity and warrants further tests in health psychology.