Play is critical in the development of cognitive and language skills in young children with autism; however, few studies have examined the impact of the intervention on the development of play skills. The current study aims to address the change in developmental skills, including play after participation in the JASPER intervention, and to examine the initiation of joint attention as a moderator of the effect of treatment on changes in play skills. The study included 109 preschool-age children who participated in the JASPER intervention and examined changes in developmental skills, including play skills. Children who received JASPER improved significantly in both play diversity (F(1,99) = 4.89, p = 0.029, ES = 0.22) and complexity (F(1,98) = 5.21, p = 0.025; ES = 0.23) compared to children in control conditions. These gains in play skills were associated with concurrent improvements in cognition and communication skills. Additionally, children with more initiations of joint attention skills at entry made greater progress in play diversity (F(1,97) = 15.85, p < 0.001 ES = 0.40) and complexity (p = 0.096). Play and joint attention skills are critical intervention targets and outcomes for children with autism.