Pediatric obesity is a significant public health concern, and the COVID-19 pandemic altered many of its risk factors. Understanding this impact can help pediatricians and public health officials prioritize initiatives and identify high-risk subgroups. We performed a retrospective longitudinal cohort study of 596 children and adolescents in a primary care clinic to determine changes in weight gain during the pandemic. A significant rise in normalized BMI was found during the prepandemic period across all age groups and initial BMI classification groups. This rate of change increased during the pandemic for those with pre-existing overweight or obesity status who were ≥10 years of age. Children with regular clinic visits in the prepandemic study period, but without a clinic visit during the pandemic, had significantly higher baseline normalized BMI and were older. These changes in the rate of weight gain during the COVID-19 pandemic period raise the possibility that pandemic influences may have exacerbated the rate of weight gain in children and adolescents with pre-existing obese or overweight range BMI. The difference in composition of the cohort without a pandemic time-period visit highlights the need for future studies in this area, especially those focused on adolescents.