Little research exists on the impact of behavior change interventions in disadvantaged communities. We conducted a prospective study to explore the effectiveness of motivational interviewing on physical activity change within a deprived community and the social- psychological and motivational predictors of change in physical activity including stage of change, self-efficacy, social support, and variables from self-determination theory and the theory of planned behavior. Five motivational interviewing counsellors recruited 207 patients and offered motivational interviewing sessions to support physical activity behavior change. At 6-months there were significant improvements in physical activity, stage of change, and social support. A dose-response relationship was evident; those who attended 2 or more consultations increased their total physical activity, stage of change and family social support more than those who attended just one. Hierarchical regression analyses indicated that number of sessions and change in stage of change predicted 28.4% of the variance in change in total physical activity and, with social support from friends, 21.0% of the variance in change walking time. Change in perceived behavioral control and attitudes, friend social support, and number of sessions predicted 16.8% of the variance in change in vigorous physical activity. Motivational interviewing is an effective approach for promoting physical activity amongst lower socio-economic status groups in the short term. The study demonstrates good translational efficacy, and contributes to a limited number of physical activity interventions targeting low income groups in the UK.