Extensive evidence and theory suggest that the development of motor skills during infancy and early childhood initiates a "developmental cascade" for cognitive abilities, such as reading and math. Motor skills are closely connected with the development of spatial cognition, an ability that supports deductive reasoning. Despite the linkage between motor skills and spatial cognition, and spatial cognition with deductive reasoning, no research has explored the developmental connection between early motor skills and reasoning ability, a plausible pathway through which the developmental cascade operates. Drawing data from the 1970 British Cohort Study (N = 1,233; 95% British, 5% other race/ethnicity; 54% male, 46% female; 7% low income, 80% middle income, 12% high income), this study investigated whether there was a relationship between gross and fine motor skills in infancy (22 months of age) and early childhood (42 months of age) and visuospatial deductive reasoning in adolescence (at 10 and 16 years of age). Results indicated that fine but not gross motor skills during early childhood positively predicted reasoning in adolescence. Critically, the fine motor-reasoning association mediated the previously observed link between early fine motor skills and adolescent reading and math ability. These results deepen our understanding of developmental cascade theory and mental model theory by identifying visuospatial reasoning (i.e., mental modeling) as a potential mechanism through which motor skills initiate cognitive development and academic success in reading and math. These findings also highlight the importance of early intervention programs targeting motor skills and illuminate the impact of those interventions on later cognitive and academic skills.