Objective This study was aimed to examine differences in infant mortality outcomes across maternal age subgroups less than 20 years in the United States with a specific focus on racial and ethnic disparities. Study Design Using National Center for Health Statistics cohort-linked live birth-infant death files (2009-2013) in this cross-sectional study, we calculated descriptive statistics by age (<15, 15-17, and 18-19 years) and racial/ethnic subgroups (non-Hispanic white [NHW], non-Hispanic black [NHB], and Hispanic) for infant, neonatal, and postneonatal mortality. Adjusted odds ratios (aOR) were calculated by race/ethnicity and age. Preterm birth and other maternal characteristics were included as covariates. Results Disparities were greatest for mothers <15 and NHB mothers. The risk of infant mortality among mothers <15 years compared to 18 to 19 years was higher regardless of race/ethnicity (NHW: aOR = 1.40, 95% confidence interval [CI]: 1.06-1.85; NHB: aOR = 1.28, 95% CI: 1.04-1.56; Hispanic: aOR = 1.36, 95%CI: 1.07-1.74). Compared to NHW mothers, NHB mothers had a consistently higher risk of infant mortality (15-17 years: aOR = 1.12, 95% CI: 1.03-1.21; 18-19 years: aOR = 1.21, 95% CI: 1.15-1.27), while Hispanic mothers had a consistently lower risk (15-17 years: aOR = 0.72, 95% CI: 0.66-0.78; 18-19 years: aOR = 0.74, 95% CI: 0.70-0.78). Adjusting for preterm birth had a greater influence than maternal characteristics on observed group differences in mortality. For neonatal and postneonatal mortality, patterns of disparities based on age and race/ethnicity differed from those of overall infant mortality. Conclusion Although infants born to younger mothers were at increased risk of mortality, variations by race/ethnicity and timing of death existed. When adjusted for preterm birth, differences in risk across age subgroups declined and, for some racial/ethnic groups, disappeared. Key Points Infant mortality risk was highest for adolescents <15 years old across racial/ethnic groups. Racial/ethnic disparities in timing of death were present even among the youngest adolescents. Infants of NHB adolescents had greatest risk of mortality, especially as age increased. Preterm birth influenced infant mortality risk, especially among NHB adolescents.