We explored how a constellation of factors affected vocabulary development of infants (N = 556; 49.6% male) from families comprising the control group of the Early Head Start Research Evaluation project. Predictors assessed at age 14 months accounted for 23.5%, 25.8%, and 30.6% of variance in receptive vocabulary (Peabody Picture Vocabulary Test) at 36 months, Pre-K, and Grade 5, respectively. Joint engagement, assessed during free play (3-bags task), and infant cognitive ability (Bayley Mental Development Index) had direct associations with vocabulary knowledge. Gestational age was indirectly associated with vocabulary knowledge via infant cognitive ability. Home environment, maternal education, maternal distress, and child gender had some direct and/or indirect effects. Latent factors were partially invariant for subgroups with Black/African American and White/European American mothers. Findings suggest a long-term impact of joint engagement on vocabulary growth at least through Grade 5, highlighting the importance of interventions to support mothers and infants living in poverty. (author abstract)