Prefrontal synthesis (PFS) is a component of constructive imagination. It is defined as the process of mentally juxtaposing objects into novel combinations. For example, to comprehend the instruction "put the cat under the dog and above the monkey," it is necessary to use PFS in order to correctly determine the spatial arrangement of the cat, dog, and monkey with relation to one another. The acquisition of PFS hinges on the use of combinatorial language during early childhood development. Accordingly, children with developmental delays exhibit a deficit in PFS, and frequent assessments are recommended for such individuals. In 2018, we developed the Mental Synthesis Evaluation Checklist (MSEC), a parent-reported evaluation designed to assess PFS and combinatorial language comprehension. In this manuscript we use MSEC to identify differences in combinatorial language acquisition between ASD (N = 29,138) and neurotypical (N = 111) children. Results emphasize the utility of the MSEC in distinguishing language deficits in ASD from typical development as early as 2 years of age (p < 0.0001).