Objective: Mother’s own milk (MOM) is the optimal feed for premature infants but may not always be sufficiently available. Alternative feeding includes donor human milk (DONOR), with or without fortification and preterm formula. This study evaluated the association between early feeding with exclusively and predominantly MOM (MAINLY-MOM) versus MOM supplemented with fortified DONOR (MOM + DONOR) or preterm formula (MOM + FORMULA) and in-hospital growth and neonatal morbidities. Method: This was a multicentre (n = 13 units) cohort study of infants born at <32 weeks’ gestation. Data captured at the point of care were extracted from the UK National Neonatal Research Database. The study groups were defined based on feeding patterns within the first 2 weeks of life using predefined cut-offs. The primary outcome was the in-hospital growth rate. Results: Data from 1,272 infants were analysed. Infants fell into two groups: extremely preterm (EPT) infants and very preterm (VPT) infants, born after <28 weeks and 28 to <32 weeks of gestation, respectively. Only 11 of 365 EPT infants received formula supplements, precluding a useful comparison of MOM + DONOR and MOM + FORMULA. There was no difference in median (25th–75th centile) growth velocity over the first 30 days of life between the MAINLY-MOM (n = 248) and MOM + DONOR (n = 106) groups: 10 (8–13) versus 10 (7–13) g/kg/day. Similarly, for VPT infants, there was no difference in growth velocities between MAINLY-MOM (n = 407), MOM + DONOR (N = 196), and MOM + FORMULA (N = 304): 11 (8–14) versus 11 (8–14) versus 11 (8–14) g/kg/day. Head growth did not differ (p value = 0.670). Cox regression analysis showed no difference in time to discharge between feeding types or any difference in major neonatal morbidities. In both EPT and VPT infants, growth velocity from the time of regaining birth weight to discharge was significantly lower in the MAINLY-MOM group compared to the MOM-DONOR group (EPT: 12.5 [11–14.2] vs. 14 [12.3–15.9] p = 0.45, VPT 13.5 [11–15.7] vs. 14.5 [12.6–16.8] p = 0.015). Conclusion: Early feeding with fortified DONOR, in comparison to formula, to supplement MOM was not associated with any differences in short-term growth, length of stay, and neonatal morbidities. However, early feeding with mainly maternal milk, compared to maternal milk supplemented with DONOR, was associated with significantly lower overall weight gain.