Communication is a key area for early intervention for pre-schoolers with autism spectrum disorder (ASD). Therefore, there is a need for reliable and valid communication assessment measures for this population. Two commonly used measures are the language scales of the Mullen Scales of Early Learning (MSEL) and the Preschool Language Scale—Fifth Edition (PLS—5). To date, limited research has compared these. The aim of the present study was to investigate the similarities and differences in scores on the two instruments for children with ASD and those who were developing typically. The MSEL and the PLS—5 assessments were administered to 49 pre-schoolers including 24 children with ASD and 25 typically developing (TD) children. Language scores on the MSEL and PLS—5 were highly correlated within each group. As expected, children with ASD performed significantly lower on both language measures compared to children who were developing typically. Children from both groups performed higher on the PLS—5 than the MSEL on the expressive language scale, and typically developing children also performed higher on the receptive language scale. Limitations and future directions for research in terms of test selection are discussed.