Background Hypertension and diabetes are associated with increased COVID‐19 severity. The association between level of control of these conditions and COVID‐19 severity is less well understood. Methods and Results This retrospective cohort study identified adults with COVID‐19, March 2020 to February 2022, in 43 US health systems in the National Patient‐Centered Clinical Research Network. Hypertension control was categorized as blood pressure (BP) <130/80, 130 to 139/80 to 89, 140 to 159/90 to 99, or ≥160/100 mm Hg, and diabetes control as glycated hemoglobin <7%, 7% to <9%, ≥9%. Adjusted, pooled logistic regression assessed associations between hypertension and diabetes control and severe COVID‐19 outcomes. Among 1 494 837 adults with COVID‐19, 43% had hypertension and 12% had diabetes. Among patients with hypertension, the highest baseline BP was associated with greater odds of hospitalization (adjusted odds ratio [aOR], 1.30 [95% CI, 1.23–1.37] for BP ≥160/100 versus BP <130/80), critical care (aOR, 1.30 [95% CI, 1.21–1.40]), and mechanical ventilation (aOR, 1.32 [95% CI, 1.17–1.50]) but not mortality (aOR, 1.08 [95% CI, 0.98–1.12]). Among patients with diabetes, the highest glycated hemoglobin was associated with greater odds of hospitalization (aOR, 1.61 [95% CI, 1.47–1.76] for glycated hemoglobin ≥9% versus <7%), critical care (aOR, 1.42 [95% CI, 1.31–1.54]), mechanical ventilation (aOR, 1.12 [95% CI, 1.02–1.23]), and mortality (aOR, 1.18 [95% CI, 1.09–1.27]). Black and Hispanic adults were more likely than White adults to experience severe COVID‐19 outcomes, independent of comorbidity score and control of hypertension or diabetes. Conclusions Among 1.5 million patients with COVID‐19, higher BP and glycated hemoglobin were associated with more severe COVID‐19 outcomes. Findings suggest that adults with poorest control of hypertension or diabetes might benefit from efforts to prevent and initiate early treatment of COVID‐19.