OBJECTIVE: To further evaluate glycemic outcomes during the observational extension phase of the Continuous Glucose Monitoring (CGM) Intervention for Teens and Young Adults randomized clinical trial (RCT). Subjects and Methods. Following a 26-week RCT comparing CGM with blood glucose monitoring (BGM) in 153 adolescents and young adults aged 14 to <25 years old with suboptimally controlled type 1 diabetes, 70 (89%) participants in the BGM group initiated use of CGM (referred to as BGM-CGM cohort), and 70 (95%) participants in the CGM group continued to use of CGM (CGM-CGM cohort) for an additional 26 weeks. RESULTS: In the CGM-CGM cohort, mean hemoglobin A1c (HbA1c) decreased from 8.9% ± 0.9% (74 ± 9.8 mmol/mol) at randomization to 8.3% ± 1.3% (67 ± 14.2 mmol/mol) at 52 weeks (p < 0.001); however, significant improvement in time in target range (TIR) 70-180 mg/dL was not observed from prerandomization (38% ± 13%) to 52 weeks (41% ± 18%). Median percent time <70 mg/dL decreased from 3.0% before randomization to 1.1% at 52 weeks (p < 0.001). In the BGM-CGM cohort, mean HbA1c decreased from 8.9% ± 1.2% (74 ± 13.1 mmol/mol) before CGM initiation to 8.5% ± 1.3% (69 ± 14.2 mmol/mol) after 26 weeks of CGM use (p < 0.001) and mean TIR increased from 34% ± 12% to 38% ± 15% (p=0.01). The median percent time <70 mg/dL decreased from 3.3% before CGM initiation to 1.2% after 26 weeks of CGM use (p < 0.001). No participants discontinued CGM use during the extension phase. CONCLUSIONS: This further evaluation of CGM supports the findings of the preceding RCT that use of CGM improves glycemic control and reduces hypoglycemia in adolescents and young adults with type 1 diabetes. This trial is registered with NCT03263494.