Cognitive assistants, such as Apple’s Siri or Amazon’s Alexa, are increasingly becoming routine elements of our daily lives, yet the effects of such devices on the humans using them has been largely unstudied. While much research has proposed new intelligent systems for specific contexts, few studies have characterized the dynamics between cognitive assistants and humans. In an effort to address this gap, this research study investigates the effect of cognitive assistants on mental workload. Participants perform the Wisconsin Card Sorting Task while working with a fake cognitive assistant (Mary) and concurrently completing a secondary speed and accuracy test. Performance with the cognitive assistant was found to be higher but with no significant difference in mental workload. Participants working with the cognitive assistant were found to be less frustrated than those completing the task without it.