The mechanisms by which high quality, two-generation early childhood services promote healthy child development remain unclear. This research examined how participation in Early Head Start (EHS), a U.S. two-generation programme for children under 3, was associated with healthy child development to age 5 through parenting processes. Using four time points of data from the Early Head Start Research and Evaluation Project, we tested a theory-driven model of direct and indirect pathways from EHS to children's language, social-emotional, and behavioural skills from age 1 to age 5 via parenting stress and practices. Results from longitudinal structural equation models for 2,555 mother-child dyads indicated that EHS participation was associated with increased responsive parenting during the very early years, improvements in children's language during the toddler years and, in turn, sustained language improvements through age 5. Long-term impacts of two-generation programmes may promote positive child outcomes in part by facilitating responsive parenting.