Background: The relationship of mean glucose measured with continuous glucose monitoring (CGM) and hemoglobin A1c (HbA1c) shows considerable variability between individuals with diabetes and may be influenced by race-related factors. Whether the relationship of mean glucose with HbA1c varies according to type 1 diabetes (T1D) or type 2 diabetes (T2D) has not been well evaluated. Methods: Data from 343 participants in four clinical trials (191 with T1D and 152 with T2D) were analyzed. Least squares linear regression models were fit with HbA1c as the dependent variable and mean glucose as the independent variable. Results: Mean age was 57 ± 15 years in the T1D cohort and 58 ± 10 years in the T2D cohort. The T1D cohort was 89% White non-Hispanic, 5% African American, 3% Hispanic, and 3% other or mixed race compared with 52%, 16%, 22%, and 9%, respectively, in the T2D cohort. The relationship between CGM-measured mean glucose and laboratory-measured HbA1c significantly differed between T1D and T2D cohorts, with HbA1c on average being higher with T2D than T1D for the same mean glucose (P = 0.002). However, this difference was largely attributable to the difference in the proportion of African Americans between T1D and T2D; and after stratifying by race, the mean glucose-HbA1c relationship showed only a small difference between T1D non-African Americans and T2D non-African Americans. The mean glucose-HbA1c relationship appeared similar for White non-Hispanic and Hispanic individuals. Conclusion: HbA1c on average was higher in T2D than T1D for a given mean glucose, but after accounting for race, there was no meaningful difference in the mean glucose-HbA1c relationship comparing T1D and T2D. The mean glucose-HbA1c relationship differs in African American compared with White individuals, but does not appear to differ comparing White non-Hispanic to Hispanic individuals. The published glucose management indicator formula appears to be suitable for both T1D and T2D.