Iron supplementation during pregnancy is associated with several health benefits, including a reduced risk of maternal anaemia and improved neonatal outcomes such as lower rates of low birth weight, infection, and anaemia in infancy. However, its impact on neonatal and post-neonatal mortality remains unclear in resource-limited settings, where adherence to maternal iron supplementation is low. This study examined the association between maternal iron supplementation and neonatal and post-neonatal mortality and explored whether low birth weight (LBW) modifies those associations. This cross-sectional study utilised Demographic and Health Survey data collected between 2015 and 2023 from 26 sub-Saharan countries, including 287,642 neonates and 279,819 post-neonates. The primary outcomes were neonatal deaths (within 28 days) and post-neonatal deaths (between 29 days and 12 months). These outcomes and the exposure variables of iron supplementation and its duration were based on maternal recall. Adjusted odds ratios (aORs) with 95% confidence intervals (CIs) were estimated using generalised linear mixed models, with stratification by LBW. There was no significant association between maternal iron supplementation and neonatal mortality (aOR = 1.07; 95% CI: 0.86, 1.34). However, the interaction between LBW and iron supplementation was statistically significant ( = 0.04). Among the LBW infants, the absence of iron supplementation increased the odds of neonatal mortality by 68% (aOR = 1.68; 95% CI: 1.14, 2.47), while supplementation for ≥90 days reduced the odds by 45% (aOR = 0.55; 95% CI: 0.35, 0.84). For post-neonatal mortality, lack of iron supplementation increased the odds by 25% (aOR = 1.25; 95% CI: 1.01, 1.56), whereas supplementation for ≥90 days reduced the odds by 27% (aOR = 0.73; 95% CI: 0.57, 0.93). Maternal iron supplementation was associated with lower post-neonatal mortality and improved neonatal survival among LBW infants. These findings suggest that iron intake may support infant survival, particularly in vulnerable populations.