Autism spectrum disorder (ASD) is a neurodevelopmental disorder, where motor impairments are common and impact individuals across the lifespan. A growing body of work suggests that alterations in predictive processes may contribute to the ASD symptomatology, such that the anticipation of upcoming events is disrupted. Such an alteration could potentially impact motor planning. Motor planning deficits are not, however, unique to ASD. Attention-deficit/hyperactivity disorder (ADHD) has been linked to motor planning and execution difficulties, and ADHD commonly co-occurs with ASD. Whether motor planning deficits are characteristic of ASD broadly or magnified in the context of co-morbid ADHD is unclear. In this study we examined the lateralized readiness potential (LRP) to assess motor preparation and action-consequence prediction. While electroencephalography (EEG) was recorded, 29 typically developing (TD) controls and 21 ASD children both with (ASD+ADHD; n=12) and without (ASD−ADHD; n=9) comorbid ADHD completed a paradigm wherein voluntary actions either did or did not result in an auditory consequence. Our results revealed differences in LRP amplitude as a function of group and whether an action produced an effect. In TD, LRP amplitude was attenuated when an action produced an effect versus when it did not. In contrast, the ASD−ADHD group demonstrated LRP enhancement when the action led to an effect. Finally, the ASD+ADHD had attenuated LRP responses compared to other groups regardless of action-effect pairings. These findings suggest that the neural mechanisms for motor preparation and prediction differ between ASD and TD, and are influenced by the presence or absence of ADHD comorbidity.