Spatial ability, particularly the cognitive capacity for mental rotations, is a critical component of human cognition. Proficiency with mental rotation tasks is linked with educational performance in various Science, Technology, Engineering, and Mathematics (STEM) disciplines, and with more general tasks such as real world wayfinding. Spatial working memory (SWM) is posited as a fundamental psychological construct associated with mental rotation ability. Through the adoption of pupillometry, this study aspired to investigate the potential role of SWM within mental rotation performance. The results of this study unexpectedly illustrated that mental effort decreased as item difficulty increased. It is posited that learning may have occurred during the initial easier tasks facilitating an increased efficiency in cognitive processing associated with SWM storage during the more difficult mental rotations tasks.