Callous-unemotional (CU) traits and effortful control (EC) are personality and temperament traits implicated in early-onset antisocial trajectories. This secondary analysis of Hitkashrut’s randomized controlled trial first tested parent training’s effects on EC and CU traits while controlling for more general treatment effects on conduct problems (CP), and subsequently tested mediation by parenting. Prekindergarten teachers in three Israeli cities identified 209 3–5 year-old (163 boys; 46 girls) preschoolers with subclinical-clinical range conduct problems. All participants were Jewish ranging from ultra-orthodox to secular. They were assigned to 14-session co-parent training groups (n = 140 couples), or to minimal intervention control groups with referral to local services as necessary (n = 69 couples). We employed averaged indices of pre- and post-intervention questionnaires completed by both parents. The testing of all hypothesized models controlled for treatment effects on CP in order to strengthen the robustness of the analyses. We found significant concurrent treatment effects on CP and on either CU traits or EC. All effects were mediated by ineffective parenting (IP): a latent variable that was indicated by negative/inconsistent practices and perceived parenting inefficacy. This is the first demonstration of parenting mediated treatment effects on both EC and CU traits in a randomized controlled study conducted in everyday practice contexts. This finding supports a disruption model of change: the reduction of IP facilitates a caregiving environment that affects children’s behavior and developing personality. The changing of personality and temperament characteristics implicated in early-onset pathways suggests an innovative prevention strategy for disruptive behavior disorders.