Working mothers exhibit high levels of inactivity, and theory-based interventions to bolster physical activity within this population are needed. This study examined the effectiveness of a brief social cognitive theory-based intervention designed to increase physical activity among working mothers. Participants (N = 141) were randomly assigned to an intervention only, intervention plus follow-up support, or waitlist control condition. The intervention consisted of two group-based workshop sessions designed to teach behavior modification strategies using social cognitive theory. Data were collected at baseline, immediately post-intervention, and 6-month follow-up. Results showed intervention participants exhibited short-term increases in physical activity, which were partially maintained 6 months later. Improvements in physical activity were mediated by increases in self-regulation and self-efficacy. This study provides some support for the effectiveness of a brief intervention to increase physical activity among working mothers. Future programs should explore alternative support mechanisms which may lead to more effective maintenance of initial behavior changes.