Online patient portals support self-management, and mobile devices expand portal access, but whether this translates to improvements in diabetes outcomes is unclear. To examine the association of adding mobile patient portal access with diabetes medication adherence and glycemic levels among adults with diabetes. This retrospective cohort study included patients with diabetes treated at Kaiser Permanente Northern California, a large, integrated health care delivery system, from April 1, 2015, to December 31, 2017. Inclusion criteria were adults with diabetes with an oral diabetes prescription at baseline and no insulin use. Data were analyzed from March 2018 to March 2019. Patient portal access status for each calendar month from April 2015 to December 2017, categorized as never used, used from a computer only, used from a mobile device only, or used from both computer and mobile device. Medication adherence, measured by monthly percentage of days covered (PDC), and glycemic levels, measured by changes in glycated hemoglobin A1c (HbA1c) levels. The association of portal access with study outcomes was assessed using linear regression with patient-level fixed effects and adjusting for time-changing variables, stratified by baseline HbA1c level. Among 111 463 included patients (mean [SD] age, 63.79 [12.93] years; 59 918 [53.76%] men), the number of patients using the portal from both a computer and mobile device increased over time from 38 371 patients (34.42%) in April 2015 to 57 920 patients (61.71%) in December 2017. Among patients with no prior portal access, adding computer-only portal access was associated with an increase in PDC of 1.16 (95% CI, 0.63 to 1.70) percentage points and a change of -0.06 (95% CI, -0.08 to -0.03) percentage points in HbA1c level, and adding both mobile and computer portal access was associated with an increase in PDC of 1.67 (95% CI, 1.10 to 2.23) percentage points and a change of -0.13 (95% CI, -0.16 to -0.10) percentage points in HbA1c level. Among patients with higher baseline HbA1c level (>8.0%), changing from no portal access to both computer and mobile access was associated with an increase in PDC of 5.09 (95% CI, 3.78 to 6.40) percentage points and a change of -0.19 (95% CI, -0.27 to -0.15) percentage points in HbA1c level. These findings suggest that providing patients with computer patient portal access and combining it with mobile patient portal access are associated with significantly improved diabetes medication adherence and glycemic control, with greater benefits among patients with more clinical need. Convenient access to portal self-management tools through a mobile device could significantly improve diabetes management.