Background Brief parent training (BPT) promotes effective parenting in parents of children with conduct problems. As previous research has provided inconsistent results, this study explored the impact of maternal mental distress on outcomes of BPT. Method Participants included 216 families randomized to BPT or a comparison group. Results Maternal distress negatively predicted five of eight outcomes, regardless of intervention allocation. Low‐maternal distress predicted lower conduct problems (parent‐ and teacher‐reported), whereas high distress predicted higher teacher‐reported conduct problems in BPT relative to comparisons. Conclusions Refinement of interventions to help children with conduct problems and distressed mothers should be prioritized.