Exposure to psychological stress and depression are associated with shorter white blood cell telomere length (TL) in adults, possibly via associated lifelong oxidative stressors. Exposure to maternal depression increases risk for future depression and behavior problems in children, and Latino youth are at high risk. Few studies have evaluated the role of exposure to maternal depression or child behavior in relation to TL in children. We assessed early-childhood exposures to maternal depression from birth to the age of 5 years and child behavior from ages 3-5 years in a cohort of Latino children in relation to child leukocyte TL at ages 4 and 5 years. Children who had oppositional defiant behavior at 3, 4 or 5 years had shorter TL than those without by ~450 base pairs (P < 0.01). In multivariate analyses, independent predictors for shorter TL at 4 and 5 years of age included oppositional defiant disorder at 3, 4 or 5 years (β = -359.25, 95% CI -633.84 to 84.66; P = 0.01), exposure to maternal clinical depression at 3 years of age (β = -363.99, 95% CI -651.24 to 764.74; P = 0.01), shorter maternal TL (β = 502.92, 95% CI 189.21-816.63) and younger paternal age at the child's birth (β = 24.63, 95% CI 1.14-48.12). Thus, exposure to maternal clinical depression (versus depressive symptoms) in early childhood was associated with deleterious consequences on child cellular health as indicated by shorter TL at 4 and 5 years of age. Similarly, children with oppositional defiant behavior also had shorter TL, possibly related to early exposures to maternal clinical depression. Our study is the first to link maternal clinical depression and oppositional defiant behavior with shorter TL in the preschool years in a relatively homogenous population of low-income Latino children.