There has been limited empirical support for interventions designed to promote physical activity targeting mobility in racially diverse older adults. This study aims to examine the effects of a Motivational Physical Activity Intervention (MPAI) on social resource, behavioral change process, physical activity, and mobility variables in sedentary older Korean Americans. A quasi-experimental, repeated-measure, pre- and post-tests design was used. Sixty-four community-dwelling, sedentary older Korean Americans (n = 33 for MPAI group, n = 31 for Attention Control group) participated in the study. There were significant improvements in social resources, including social support from family and friends; behavioral change process variables, including self-efficacy; motivational appraisal; and self-regulation for physical activity. There were significant intervention effects on physical activity, walking endurance, and flexibility. The MPAI is supported as improving mobility and physical activity, as well as increasing motivation for physical activity in older Korean Americans.