The use of social robots in therapy for children with autism has been explored for more than 20 years, but there still is limited clinical evidence. The work presented here provides a systematic approach to evaluating both efficacy and effectiveness, bridging the gap between theory and practice by targeting joint attention, imitation, and turn-taking as core developmental mechanisms that can make a difference in autism interventions. We present two randomized clinical trials with different robot-assisted therapy implementations aimed at young children. The first is an efficacy trial (n = 69; mean age = 4.4 years) showing that 12 biweekly sessions of in-clinic robot-assisted therapy achieve equivalent outcomes to conventional treatment but with a significant increase in the patients’ engagement. The second trial (n = 63; mean age = 5.9 years) evaluates the effectiveness in real-world settings by substituting the clinical setup with a simpler one for use in schools or homes. Over the course of a modest dosage of five sessions, we show equivalent outcomes to standard treatment. Both efficacy and effectiveness trials lend further credibility to the beneficial role that social robots can play in autism therapy while also highlighting the potential advantages of portable and cost-effective setups.