Globally, an estimated 15 million children are born prematurely each year, resulting in a high burden of under-five mortality and neurodevelopmental disability. Kangaroo Mother Care (KMC) is a key intervention to support the development of preterm infants. However, evidence on the impact of KMC in routine care settings remains limited. This paper examines the associations between maternal KMC efforts and child development among preterm infants using data from a prospective cohort study conducted in São Paulo, Brazil. Study outcomes were height-for-age z-scores (HAZ) and cognitive development at age 3 years. Practicing KMC was positively associated with HAZ (+0.91 SD, 95% CI [0.126, 1.695]), and cognition (+0.37 SD, [0.034, 0.806]) of preterm children. The results suggest that KMC can improve children's cognitive and physical development. New programmes to increase KMC uptake in the setting studied may be beneficial for both mothers and their preterm children.