Social/emotional skills in early childhood are associated with education, labor market, and family formation outcomes throughout the life course. One explanation for these associations is that poor social/emotional skills in early childhood interfere with the development of cognitive skills. In this paper, we use data from the Fragile Families and Child Wellbeing Study (N = 2,302) to examine how the timing of social/emotional skills—measured as internalizing, externalizing, and attention problem behaviors in early childhood—is associated with cognitive test scores in middle childhood. Results show that externalizing problems at age 3 and attention problems at age 5, as well as externalizing and attention problems at both ages 3 and 5, are associated with poor cognitive development in middle childhood, net of a wide array of control variables and prior test scores. Surprisingly, maternal engagement at age five does not mediate these associations.