This longitudinal study examined how receptive and expressive vocabulary assessments capture vocabulary development in children with Autism Spectrum Disorder (ASD) and typically developing (TD) children. Using mixed regression modelling, we explored when children with ASD significantly different from TD children. We also examined the variability of individual trajectories of vocabulary development in children with ASD. Children with ASD showed slowed trajectories and significantly differed from TD children by 24 months on all assessments except for picture-based assessments. Children with ASD also showed high heterogeneity in trajectories, with some showing inconsistent patterns of growth, stagnation, and regression across assessments. This suggests that conclusions based on individual assessments of vocabulary can vary and assessment characteristics must be considered when monitoring vocabulary development.