Abstract Background Peer support provides varied health benefits, but how it achieves these benefits is not well understood. Purpose Examine a) predictors of participation in peer support interventions for diabetes management, and b) relationship between participation and glycemic control. Methods Seven peer support interventions funded through Peers for Progress provided pre/post data on 1,746 participants’ glycemic control (hemoglobin A1c), contacts with peer supporters as an indicator of participation, health literacy, availability/satisfaction with support for diabetes management from family and clinical team, quality of life (EQ-Index), diabetes distress, depression (PHQ-8), BMI, gender, age, education, and years with diabetes. Results Structural equation modeling indicated a) lower levels of available support for diabetes management, higher depression scores, and older age predicted more contacts with peer supporters, and b) more contacts predicted lower levels of final HbA1c as did lower baseline levels of BMI and diabetes distress and fewer years living with diabetes. Parallel effects of contacts on HbA1c, although not statistically significant, were observed among those with baseline HbA1c values > 7.5% or > 9%. Additionally, no, low, moderate, and high contacts showed a significant linear, dose–response relationship with final HbA1c. Baseline and covariate-adjusted, final HbA1c was 8.18% versus 7.86% for those with no versus high contacts. Conclusions Peer support reached/benefitted those at greater disadvantage. Less social support for dealing with diabetes and higher PHQ-8 scores predicted greater participation in peer support. Participation in turn predicted lower HbA1c across levels of baseline HbA1c, and in a dose–response relationship across levels of participation.