Background: The One Drop vertical bar Mobile app supports manual and passive (via HealthKit and One Drop's glucose meter) tracking of self-care and glycated hemoglobin A(1c) (HbA(1c)). Objective: We assessed the HbA(1c) change of a sample of people with type 1 diabetes (T1D) or type 2 diabetes (T2D) using the One Drop vertical bar Mobile app on iPhone and Apple Watch, and tested relationships between self-care tracking with the app and HbA(1c) change. Methods: In June 2017, we identified people with diabetes using the One Drop vertical bar Mobile app on iPhone and Apple Watch who entered two HbA(1c) measurements in the app 60 to 365 days apart. We assessed the relationship between using the app and HbA(1c) change. Results: Users had T1D (n=65) or T2D (n=191), were 22.7% (58/219) female, with diabetes for a mean 8.34 (SD 8.79) years, and tracked a mean 2176.35 (SD 3430.23) self-care activities between HbA(1c) entries. There was a significant 1.36% or 14.9 mmol/mol HbA(1c) reduction (F=62.60, P<.001) from the first (8.72%, 71.8 mmol/mol) to second HbA(1c) (7.36%, 56.9 mmol/mol) measurement. Tracking carbohydrates was independently associated with greater HbA(1c) improvement (all P<.01). Conclusions: Using One Drop vertical bar Mobile on iPhone and Apple Watch may favorably impact glycemic control.