Maternal supportiveness protects against negative outcomes in at-risk children. The present study sought to understand whether there are multiple trajectories (classes) of maternal supportiveness across toddlerhood, whether supportiveness classes relate to unique child outcomes in middle childhood dependent race/ethnicity, and whether early childhood emotion regulation mediates these relations in a low-income sample. To address our objectives, 2,977 mother-child dyads from the Early Head Start Research and Evaluation Project were examined. Maternal supportiveness was assessed at 14, 24, and 36 months, emotion regulation was assessed at 36 months, and mothers' and children's perceptions of internalizing and externalizing behavior were assessed during fifth grade. Growth mixture modeling revealed four classes: high and stable, moderate and increasing, moderate and decreasing, and low and stable. Ultimately, emotion regulation mediated the effects of class on child outcomes for all outcomes except parent-reported internalizing behavior. Although children whose mothers were high and stable in supportiveness showed the least negative outcomes, those whose mothers were more moderate in supportiveness showed protection from negative outcomes when compared to those whose mothers were low in supportiveness. Race/ethnicity played a significant role in the mothers' perceptions of their child's problem behaviors. Results elucidate protective patterns of maternal supportiveness that distinctly buffer at-risk children from negative outcomes and can inform prevention/intervention design.