Theoretical models of Attention deficit/hyperactivity disorder (ADHD) have long implicated executive function (EF) skills as contributing to the etiology, maintenance, and changes in ADHD symptomatology over time. Although there is interest making within-person inferences (i.e., deficits in EF skills give rise to ADHD behaviors), most of the evidence has been derived from studies that conflated between- and within-person sources of variance. Here, we use repeated-measures data to test within-person association between EF skills and ADHD behaviors. Participants included 1160 children from the Family Life Project, an ongoing prospective longitudinal study of child development in low-income, nonmetropolitan communities. We tested the magnitude of the association between EF skills and ADHD behaviors when children were 3, 4, and 5 years old. Consistent with meta-analyses, unadjusted bivariate associations between EF and ADHD (which reflect combined between- and within-person variation) were of moderate magnitude (rs = −0.20 to −0.30). However, after controlling for all time-invariant, between-person sources of variation, the within-person associations between EF skills and ADHD behaviors were weak (βs − 0.04 to −0.05, ps = 0.01). These results suggest that EF skills may contribute less prominently to ADHD behaviors in early childhood than is commonly assumed and provoke broader questions about developmental models of ADHD.