Although there is growing interest in telehealth to deliver parent-mediated intervention for autistic children, empirical evaluations are limited, and little is known regarding the relative benefits of self-directed and therapist-assisted telehealth interventions. This study examined the effect of self-directed and therapist-assisted ImPACT Online on parent learning and well-being, moderators of treatment, and predictors of program engagement. Sixty-four young autistic children and their primary caregiver participated. Children were matched on age and developmental quotient and randomly assigned to a therapist-assisted, self-directed, or resource support control group. Participants were assessed at intake, after 6 months (post), and at a 3-month follow-up. There was a significant treatment effect for parent learning for the therapist-assisted but not self-directed program; when analysis was limited to parents who completed the program, treatment effects were observed for both groups. There were no treatment effects for parent self-efficacy or parenting stress; however, there was an effect on parents' perception of their child's positive impact. Parenting stress did not moderate the effect of group on parent outcomes. Parent age, program satisfaction, and therapist assistance were all significant predictors of parent program engagement. This study supports the efficacy of therapist-assisted telehealth parent-mediated intervention for teaching parents intervention strategies to support their child's social communication and improving their perceptions of their child's positive impact, and suggests that self-directed programs may be beneficial for parents who fully engage with the program.