Background: Differences in how developmental pathways interact dynamically in children with autism spectrum disorder (ASD) likely contribute in important ways to phenotypic heterogeneity. This study aimed to model longitudinal reciprocal associations between social competence (SOC) and language (LANG) pathways in young children with ASD. Methods: Data were obtained from 365 participants aged 2-4 years who had recently been diagnosed with an ASD and who were followed over three time points: baseline (time of diagnosis), 6- and 12 months later. Using structural equation modeling, a cross-lagged reciprocal effects model was developed that incorporated auto-regressive (stability) paths for SOC (using the Socialization subscale of the Vineland Adaptive Behavior Scales-2) and LANG (using the Preschool Language Scale-4 Auditory Comprehension subscale). Cross-domain associations included within-time correlations and lagged associations. Results: SOC and LANG were highly stable over 12 months. Small reciprocal cross-lagged associations were found across most time points and within-time correlations decreased over time. There were no differences in strength of cross-lagged associations between SOC-LANG and LANG-SOC across time points. Few differences were found between subgroups of children with ASD with and without cognitive impairment. Conclusions: Longitudinal reciprocal cross-domain associations between social competence and language were small in this sample of young children with ASD. Instead, a pattern emerged to suggest that the two domains were strongly associated around time of diagnosis in preschoolers with ASD, and then appeared to become more independent over the ensuing 12 months.