Emerging research shows that Acceptance and Commitment Therapy (ACT) may improve mental health for caregivers. Parents of autistic children, adolescents, and adults (N = 54) were randomly assigned to either complete a brief group-based ACT intervention or remain on the waitlist. Participants completed surveys immediately prior to randomization, and 3-, 7-, and 17-weeks post-randomization. The primary outcome was depression symptoms and secondary outcomes included stress, goal attainment, positive affect, ACT psychological processes, child mental health, and family functioning. Mixed effects linear models testing Group × Time interaction indicated the Treatment group (n = 27) demonstrated greater post-intervention improvements than the Waitlist group (n = 27) in parent depression (p = 0.03, d = -0.64) and family distress (p = 0.04, d = -0.57). Treatment group parents also reported greater short-term gains in positive affect (p =< 0.001, d = 0.77) and personal goal attainment (p = 0.007, d = 0.80), compared to the Waitlist group. Although there was no significant Group × Time interaction for other outcomes, stress (b = -2.58, p = 0.01), defusion (b = -3.78, p = 0.001), and experiential avoidance (b = -4.22, p = 0.01) showed improvement for the Treatment group, but not the Waitlist group, at post-intervention. All Treatment group improvements were maintained at follow-up. Results suggest that a brief ACT group intervention is efficacious for improving some aspects of mental health for parents of autistic children.