Mothers have lower moderate-to-vigorous physical activity than non-mothers and this negatively impacts health. Group dynamics interventions have been successful for increasing physical activity, yet studies using these strategies among mothers are limited and rely on self-reported physical activity. The goal of this study was to examine the effectiveness of a collective-efficacy enhanced physical activity intervention that used group dynamics principles compared to a standard group-based intervention. Mothers ( = 86) were randomly assigned to a 6-week intervention (collective-efficacy enhanced) or standard health education/walking group. Main outcome measures: participants wore pedometers for 1-week and completed a self-efficacy questionnaire at pre-intervention, post-intervention, and 3-month follow-up. There were statistically significant increases in activity minutes ( = 0.05) and step counts ( = 0.049) and no changes in self-efficacy ( = 0.74) from pre- to post-intervention. There were no group by time interactions for changes in physical activity or self-efficacy. None of the changes in physical activity were maintained at follow-up. There is a continued need to understand how to use social factors to promote sustained physical activity among mothers.