Enhancing immediate and contingent responding by caregivers to children's signals is an important strategy to support social interactions between caregivers and their children with autism. Yet, there has been limited examination of parents' responsive behaviour in association with children's social behaviour post caregiver-mediated intervention. Eighty-five dyads were randomized to one of two 10-week caregiver-training interventions. Parent-child play interactions were coded for parental responsivity and children's joint engagement. Significant gains in responsivity and time jointly engaged were found post JASPER parent-mediated intervention over a psychoeducation intervention. Further, combining higher levels of responsive behaviour with greater adoption of intervention strategies was associated with greater time jointly engaged. Findings encourage a focus on enhancing responsive behaviour in parent-mediated intervention models.