Introduction Measurement of glucose levels is the mainstay method of ensuring good glycemic control and preventing complications associated with uncontrolled diabetes. Continuous glucose monitoring enables easy and effective monitoring of interstitial glucose around the clock and hence improves glycemic control. Objectives This study aimed to measure the effect of continuous glucose monitoring on glycated hemoglobin (HbA1c) at 3, 6, and 9 months following sensor insertion. Methods A retrospective cohort study of pediatric and adolescent type 1 diabetes mellitus patients randomly sampled from 32 Ministry of Health diabetes centers across Saudi Arabia was performed. Patients were subjected to flash glucose monitoring using the FreeStyle ® Libre flash glucose monitoring system (Abbott Diabetes Care, Witney, UK), an intermittently scanned continuous glucose monitoring device approved by the Conformité Européenne in 2014. These patients were first-time users of any kind of continuous glucose monitoring system, aged 4–18 years, and received insulin via multiple dose injection or continuous subcutaneous insulin infusion for at least 6 months prior to study start. Patients were excluded if they had used flash glucose monitoring or other interstitial glucose monitoring systems in the past 3 months, were pregnant, or had existing hemoglobinopathies. The flash glucose monitoring sensor was attached to the back of the upper arm at the baseline visit. HbA1c (%) was measured at baseline and 3, 6, and 9 months. Patient demographics were collected from electronic health records. Results 1,307 patients were included, with a mean age of 11.1 years (standard deviation 3.6 years). Where specified, 51.4% were female. Mean HbA1c significantly reduced from baseline (10.8%) to 3 months (9.8%, p < 0.001), 6 months (9.2%, p < 0.001), and 9 months (9.1%, p < 0.001). For individuals with baseline HbA1c > 9%, mean HbA1c was significantly reduced from baseline (11.7%) to 3 months (10.3%, p < 0.001), 6 months (9.6%, p < 0.001), and 9 months (9.5%, p < 0.001). Conclusions Flash glucose monitoring significantly reduced HbA1c levels at 3, 6, and 9 months following sensor insertion. This reduction was greatest in those patients with higher HbA1c at baseline (> 9%).