Innovative, culturally tailored strategies are needed to extend diabetes education and support efforts in lower-resourced primary care practices serving racial/ethnic minority groups. A randomized controlled trial (RCT) examined the effect of a diabetes self-care coaching intervention delivered by medical assistants and the joint effect of intervention and ethnicity over time. The randomized repeated-measures design included 270 low-income African American and Hispanic/Latino patients with type 2 diabetes. The 1-year clinic- and telephone-based medical assistant coaching intervention was culturally tailored and guided by theoretical frameworks. A1C was obtained, and a self-care measure was completed at baseline, 6 months, and 12 months. Data were analyzed using mixed-effects models with and without adjustment for covariates. There was a significant overall improvement in mean self-care scores across time, but no intervention effect. Results revealed differences in self-care patterns across racial/ethnic subgroups. No differences were found for A1C levels across time or group.