Maltreated children are susceptible to dysregulation, but developmental mechanisms at the family level that influence this process are understudied. In the current investigation, 4 mediators (positive parenting, positive and negative family expressiveness, and maternal sensitive guidance during reminiscing) were examined as process variables through which maltreatment relates to 2 dimensions of child emotional self-regulation (adaptive emotion regulation and lability/negativity) measured across 3 time points (baseline, 2 months, and 6 months later) using longitudinal mediation analysis with latent growth modeling. These processes were evaluated in the context of a randomized controlled trial of a brief intervention aimed at improving maternal sensitive guidance during reminiscing. Participants included 160 maltreating mothers randomized into intervention (n = 81) or control intervention (n = 79) conditions and 78 demographically matched, nonmaltreating mothers and their 3- to 6-year-old children (N = 238). In the primary analysis, maternal sensitive guidance at baseline mediated relations between early maltreatment and emotion regulation and lability/negativity at 6 months, and latent change in emotion regulation across the 3 time points. Additionally, the intervention predicted steeper positive change in emotion regulation. In the secondary analysis, there was evidence of indirect effects of the intervention on emotional self-regulation through maternal sensitive guidance, positive parenting, and positive family expressiveness. Implications and future directions are discussed.