Background Maternal age and cohabitation (women living with a partner in marriage or in a common-law relationship) are known to be associated with adverse birth outcomes. However, how these two factors jointly contribute to birth outcomes is not well understood, particularly among minor mothers. Methods All live births that occurred in Argentina 2001–2021 (N=13 807 028) were used to estimate the prevalence of births to minor mothers (<18 years). In analyses restricted to mothers aged ≤24 years (N=5 159 231), multinomial and binary logistic models were used to obtain crude and adjusted ORs (aOR) for the joint association between cohabitation status and maternal age groups (minors: ≤15 and 16–17 years, and young adults: 18–19 years and 20–24 years) with preterm birth (PTB), small-for-gestational age (SGA) groups and repeat birth. Results Minor mothers accounted for 6% of all births (n=791 731), lived in poor regions and were more likely to have incomplete primary education and no employment. Among mothers aged <24 years, adverse outcomes jointly varied according to cohabitation status and maternal age (p value for interaction <0.001 in all models). Adverse outcomes were more frequent among minors. Compared with non-cohabiting mothers aged 20–24 years, cohabiting mothers aged 20–24 years had lower odds of very PTB (0.82% vs 1.19%), moderately PTB (7.15% vs 6.33%), extreme SGA (1.98% vs 2.56%) and moderately SGA (3.63% vs 4.48%, respectively). However, compared with non-cohabiting mothers aged 20–24 years, cohabiting minor mothers, particularly those aged ≤15 years had higher odds of very PTB (24–31 gestation weeks) (AOR: 1.86 (95% CI 1.76, 1.97)), moderately PTB (32–36 weeks) (AOR: 1.53 (95% CI 1.49, 1.57)), extreme SGA (<3rd percentile) (AOR: 1.10 (95% CI 1.06, 1.14)) and moderately SGA (3rd to <10th percentile) (AOR: 1.05 (95% CI 1.01, 1.08)). Conclusions The cohabitation advantage among young adults was not observed among minor mothers, particularly those aged ≤15 years.