Objective: In this study, the aim was to (a) test for the first time whether implementation intentions formed outside the laboratory can overcome the effects of habits, and (b) see whether the operation of implementation intentions could be improved by asking people to form certain “when–then” plans as opposed to uncertain “if–then” plans. Method: The study employed a 2 × 2 fully factorial design with baseline and follow-up measures of smoking status and habits. Smokers (N = 168; circa 33 years of age; 79 women, 89 men) were randomly allocated to 1 of 2 intervention groups to form either if–then plans or when–then plans using supporting tools, or to 1 of 2 control conditions in which they were exposed to identical supporting tools but were not asked to form if–then plans or when–then plans. Results: Certainty did not affect the operation of implementation intentions, but smokers who formed implementation intentions were significantly more likely to quit, χ2(1, N = 168) = 8.86, p < .01, and the effect was mediated by changes in smoking habits (95% CI [0.02, 0.14]). Similar effects were observed when cigarettes smoked per day, nicotine dependence, and craving served as the dependent variables. Conclusion: The findings demonstrate that people who have formed implementation intentions can overcome habits, such as smoking, outside the laboratory. The supporting tools described in the present research could be deployed at low cost with high public health reach to support behavior change.