This study examined an insufficiently studied predictor of functional language, social motivation, in a group of 87 young children with autism spectrum disorder. Participants (age 14-31 months at the start of the study) were assessed at four times over 24 months. We tested total and indirect associations between early social motivation and later expressive language to understand the behavioral mechanisms by which social motivation might affect functional language development. Results from this study show that early social motivation predicts functional language 2 years later in young children with autism spectrum disorder. In addition, this positive longitudinal association is serially mediated by intentional communication and then receptive language. These findings are consistent with the elicited bootstrapping hypothesis which proposes that children with relatively stronger social motivation produce more intentional communication. This, in turn, elicits others' linguistic input which impels receptive language development. This process eventually results in relatively increased expressive language development.