Adults with type 2 diabetes mellitus (T2DM) often receive self-management support from adult children, siblings or close friends residing outside of their home. However, the role of out-of-home support in patients’ self-management and well-being is unclear. Patients (N = 313) with HbA1c > 7.5% were recruited from community primary care clinics for a mobile health intervention trial and identified an out-of-home informal support person, herein called a CarePartner: 38% also had an in-home supporter. We tested cross-sectional adjusted associations between CarePartner relationship characteristics and patients’ self-management, diabetes distress, and HbA1c and whether having an in-home supporter modified these associations. Greater CarePartner closeness was associated with a greater odds of perfect medication adherence (AOR = 1.19, p = .029), more fruit/vegetable intake (β = 0.14, p = .018), and lower diabetes distress (β = − 0.14, p = .012). More frequent CarePartner contact was associated with better HbA1c among patients with an in-home supporter but with worse HbA1c among patients without an in-home supporter (interaction β = − 0.45, p = .005). Emotional closeness with a CarePartner may be important for supporting T2DM self-management and reducing diabetes distress. CarePartners may appropriately engage more frequently when patients with no in-home supporter have poorly controlled diabetes.