Parents assessed the development of 12,081 two- to six-year-old children with Autism Spectrum Disorder (ASD) quarterly for three years on five subscales: combinatorial receptive language, expressive language, sociability, sensory awareness, and health. Longer duration of time spent with an adult actively involved in the same activity (termed here joint engagement) was associated with improved trajectory of combinatorial receptive language, expressive language, and sensory awareness. On the annualized basis, the high-joint-engagement group (3 h or more of joint engagement per day) improved their combinatorial receptive language 1.4-times faster (p = 0.0019), expressive language 1.5-times faster (p < 0.0001), and sensory awareness 1.5-times faster (p = 0.0248) than the low-joint-engagement group (1 h or less joint engagement per day). The difference in the sociability and the health scores at the end of 3-year study was insignificant. This study confirms the importance of autistic children spending more time with adults actively involved in the same activity and highlights the need to include joint engagement as a target for intervention with this population.