Background: Sociodemographic and structural inequities shape the risk of alcohol-related harms among underage individuals. This study assessed disparities in alcohol-related emergency department (ED) visits, using the COVID-19 pandemic as a contextual backdrop to examine how these inequities shifted during a period of widespread disruption. Methods: This repeated cross-sectional study used pooled Nationwide Emergency Department Sample (NEDS) data (January 2019–December 2021) for individuals aged 12–20, comparing pre-COVID-19 (January 2019–March 2020) and during-COVID-19 (April 2020–December 2021) periods. Alcohol-related visits were identified via ICD-10-CM codes. Survey-weighted multivariable logistic regression estimated adjusted odds ratios (ORs) and 95% confidence intervals (CIs), adjusting for demographic, socioeconomic, and geographic factors, with interaction terms assessing period-related changes in disparities. Results: Sociodemographic disparities were the primary drivers of alcohol-related ED visits. Native American youth had markedly higher odds than White youth (OR = 3.68, 95% CI: 3.52–3.86), while Black, Hispanic, and Asian youth had lower odds. Uninsured (OR = 1.43) and charity-care youth also showed elevated odds. COVID-19 itself was not significantly associated with overall visits (OR = 1.03, p = 0.053), but disparities widened for Native American youth (OR = 1.20) and narrowed modestly for Black (OR = 1.04) and Medicaid-insured (OR = 1.05) youth, while other disparities remained unchanged. Conclusions: Persistent structural disparities, particularly by race/ethnicity and insurance status, drive underage alcohol-related ED utilization far more than the pandemic itself, underscoring the need for equity-focused prevention strategies targeting systemic barriers to care.