Mentoring-based interventions show promise among children in foster care, but previous research suggests that some benefit more than others. Because children in foster care experience relationship disruptions that could affect mentoring effectiveness, we examined whether children’s relational histories at baseline (i.e., relationship quality with birth parents, relationship quality with foster parents, caregiver instability, and previous mentoring experience) moderated the impact of a mentoring intervention on children’s mental health, trauma symptoms, and quality of life. Participants included 426 racially and ethnically diverse children (age: 9–11; 52% male) who participated in a randomized controlled trial of the Fostering Healthy Futures program (FHF), a 9-month one-to-one mentoring and skills group intervention. Results showed that relationship quality with foster parents and prior mentoring experience did not moderate intervention impact. Relationship quality with birth parents and caregiver instability pre-program, however, moderated the effect on some outcomes. The impact on quality of life was stronger for children with weaker birth parent relationships and fewer caregiver changes. Likewise, the impact on trauma symptoms was stronger for those with fewer caregiver changes. Overall, FHF seems to positively impact children with varied relational histories, yet some may derive more benefits – particularly those with fewer caregiver changes pre-program.