Background and Objectives: Donor human milk (DHM) from a human milk bank (HMB) is used to feed low-birthweight (LBW) and preterm infants when mothers cannot provide their own breastmilk. The misuse of DHM could interfere with mothers’ breastmilk and weaken breastfeeding efforts. This study aimed to identify factors behind prolonged DHM usage during the first six years of Vietnam’s first HMB. Methods: Data were extracted from the Da Nang HMB’s digital monitoring system. We defined prolonged DHM use as four or more days in the neonatal unit and two or more days in postnatal wards. Results: Over six years, 25,420 infants received DHM, with 45.3% of the infants being female, 54.7% being male, 70.0% being born via cesarean section, and 77.2% being full-term. In the neonatal unit (n = 7001), 38.0% of infants used DHM for ≥4 days. Adjusted odds ratios (aORs) for prolonged use were 0.14 for infants weighing <1000 g, 0.78 for infants weighing 1000–<1500 g, and 0.67 for infants weighing ≥2000 g (p < 0.01), compared to those weighing 1500–<2000 g. Compared to gestational ages of 32–<34 weeks, the aORs were 0.26 for <28 weeks, 0.71 for 34–<37 weeks, and 0.35 for ≥37 weeks (p < 0.01). In postnatal wards (n = 18,419), 53.1% of infants used DHM for ≥2 days. Compared to term, normal-weight infants, the aORs were 1.25 for LBW–preterm, 1.17 for LBW–term, and 1.21 for normal-weight–preterm infants (p < 0.05). Prolonged DHM use was associated with cesarean births in neonatal units (aOR 2.24, p < 0.01) and postnatal wards (aOR 1.44, p < 0.01). Conclusions: DHM is used briefly to bridge nutritional gaps and transition to mothers’ breastmilk, but LBW, preterm births, and cesarean births are linked to prolonged use. Healthcare providers should support those at risk of prolonged DHM use and prioritize reducing unnecessary cesarean births.