This study explored the effects of mindfulness and implementation intentions in increasing physical activity and reducing BMI. Twelve males and 48 females who were attempting to get fit were randomised to one of three comparison groups. Two of these groups acted as intervention groups. The first employed strategies to encourage both mindfulness and implementation intentions and the second targeted implementation intentions only. The remaining group acted as a ‘no strategy’ control. Participants in the intervention groups received two 5–20 min training sessions (1 week apart). Participants in the mindfulness group were also asked to practice mindfulness strategies for 5 min every day. The primary outcome was level of physical activity and secondary outcomes were BMI and body composition. To examine potential mediators and moderators participants also completed measures of personality, attentional control, mindfulness and self-regulatory ability. Measures were taken at baseline, 1 month and 6 months. A series of 3 (group) × 2 (time) mixed ANOVAs were used to examine the effects of the interventions on physical activity, BMI and body composition. Results showed no significant interactions between group and time between baseline and 6 months suggesting that neither intervention strategies were effective at promoting change during these periods. The findings highlight the difficulty of bringing about sustained change using very brief intervention strategies.