Robust evidence supports the efficacy of early autism intervention. Despite broad consensus that earlier intervention leads to better outcomes, evidence for this has been limited to correlational studies. This study examined timing effects of the Early Social Interaction (ESI) model, a parent-implemented intervention, using a complete crossover randomized controlled trial (RCT). Effects of Individual-ESI were compared when initiated at 18 or 27 months of age, and also contrasted with effects of the less intensive Group-ESI as an active control condition. Participants included 82 autistic toddlers who received 9 months of Individual-ESI "and" 9 months of Group-ESI, with the timing/order randomized. Blinded clinicians completed assessments at baseline (18 months of age), end of Condition 1 (27 months), and end of Condition 2 (36 months). Toddlers randomized to Individual-ESI at 18 months showed greater gains during treatment than those starting Individual-ESI at 27 months in receptive/expressive language, social communication, and daily living skills. This pattern was not observed for Group-ESI, demonstrating that timing effects were specific to Individual-ESI and ruling out maturation effects. This RCT demonstrated that earlier intensive, individualized intervention led to greater improvements, and suggests that even a narrow window of 18 versus 27 months may impact child outcomes.