Despite widespread use of the Mullen Scales of Early Learning (MSEL; E. M. Mullen, 1995) as a cognitive test for children with autism spectrum disorders and other developmental disabilities, the instrument has not been independently validated for use in these populations. Convergent validity of the MSEL and the Differential Ability Scales (DAS; C. D. Elliott, 1990, 2007) was examined in 53 children with autism spectrum disorder and 19 children with nonspectrum diagnoses. Results showed good convergent validity with respect to nonverbal IQ (NVIQ), verbal IQ (VIQ), and NVIQ–VIQ profiles. These findings provide preliminary support for the practice of using MSEL age-equivalents to generate NVIQ and VIQ scores. Establishing convergent validity of cognitive tests is needed before IQs derived from different tests can be conceptualized as a uniform construct.