Introduction: Body composition, specifically fat-free mass (FFM), of preterm infants is associated with improved neurodevelopmental outcomes. Little is known about body composition of preterm infants after discharge. Preterm body composition was measured by air displacement plethysmography (ADP) at two time points, inpatient (35–40 weeks postmenstrual age [PMA]) and outpatient (48–58 weeks PMA), with neonatal factors and neurodevelopmental testing at 4–6 months corrected age. We hypothesized increased FFM is positively associated with neurodevelopment. Methods: From 2007 to 2011, 510 infants admitted to the Medical University of South Carolina’s neonatal intensive care unit underwent ADP. A total of 379 of 510 (74%) had anthropometrics at birth, an ADP scan with FFM, fat mass, fat percent z-scores, and an outpatient neurodevelopmental evaluation (CAT/CLAMS, Peabody Gross Motor). Variables were compared using multivariate analyses for body composition measurements. Results: The infants were 32 ± 4.8 weeks gestational age at birth with an average birth weight of 1,697 ± 932 g. Most (56%) infants received maternal milk at discharge. CAT, CLAMS, and gross motor scores had positive correlations with FFM z-scores at inpatient and outpatient ADP (p < 0.05). Receiving maternal milk at discharge was positively associated with cognitive (β = 0.22, p < 0.05) and language scores (β = 0.26, p < 0.05). Conclusion: Increased FFM is associated with improved cognitive, language, and gross motor testing. Maternal milk was positively associated with language and cognitive scores.