In U.S. children of ages 2-5 years, combined overweight and obesity has increased to 21%, with African American children of this age range highest at 26%. Lack of physical activity is highly predictive of overweight and obesity in children. Preschools may be a useful point for intervention. An innovative preschool physical activity treatment (Start For Life) was developed based on principles of social cognitive and self-efficacy theory. It incorporated 30 minutes daily of highly structured physical activity with behavioral and self-regulatory skills training (e.g. goal setting, self-monitoring, productive self-talk) interspersed. Data obtained from accelerometry was used to contrast physical activity outputs during the preschool day in the Start For Life condition (n = 202) with a usual-care control condition (n = 136). After controlling for age and sex of the primarily African American participants (M age = 4.7 years), changes over eight weeks in moderate-to-vigorous and vigorous physical activity were significant, and significantly more favorable in the Start For Life group; F(1, 344) = 4.98, p = .026 and F(1, 344) = 3.60, p = .058, respectively. Start For Life was associated with a weekly increase in moderate-to-vigorous physical activity of approximately 40 minutes. After sufficient replications that better account for different sample types, parental effects and physical activity outside of the school day, and long-term effects, widespread dissemination may be considered.