The psychological assessment is an important component of the diagnostic evaluation in young children suspected of having an Autism Spectrum Disorder but can be hampered by behavioral difficulties. Overt behaviors during administration of the Mullen Scales of Early Learning were coded in 22 preschoolers with an Autism Spectrum Disorder and 20 age-matched typically developing children. Children in the Autism Spectrum Disorder group required less time to complete the assessment but spent proportionally more time exhibiting off-task behaviors and less time engaged with the assessment. Scores obtained on the Mullen Scales were positively correlated with level of engagement and negatively correlated with off task behaviors.