Objectives Undernutrition during early life contributes to more than 200 million children globally not fulfilling their developmental potential. Our objective was to determine whether dietary supplementation with several formulations of lipid-based nutrient supplements (LNS), which differed in dose per day and milk content, positively affect infant development in Malawi. Methods We randomly assigned 1932 infants age 6 months to receive one of the following for 12 months: 10, 20 g, or 40 g/day milk-containing LNS, 20 g or 40 g/day milk-free LNS, or no supplement until 18 months of age (control group). We assessed motor, language, socio-emotional, and executive function at age 18 months. Primary analysis was by intention-to-treat and we also examined 13 potential effect modifiers, including the child’s initial nutritional status and level of developmental stimulation. The study is registered as clinical trial NCT00945698. Results We found no significant differences between intervention groups in any scores. The difference in mean z-scores between children in the control group and children in the intervention groups ranged from −0.08 to 0.04 for motor development (p = 0.76), −0.05 to 0.01 for language development (p = 0.97), −0.15 to 0.11 for socio-emotional development (p = 0.22), and −0.02 to 0.20 for executive function (p = 0.24). We did not find that initial nutritional status, developmental stimulation, or other factors modified the effect LNS versus control group. Conclusions for Practice Our results suggest that in a population such as this one, provision of LNS from age 6 to 18 months would not affect motor, language, socio-emotional, or executive function skills at age 18 months.