This study tested whether the bivariate association between simple reaction time (SRT) and executive function (EF) performance that has been observed in early childhood represented a between- and/or within-person association. Up to three repeated assessments (i.e., fall, winter, and spring assessments from September to May) were available for 282 preschool-aged children (Mage = 4.2 years; 54% female) who participated in the Kids Activity and Learning Study. A series of three-level hierarchical linear models (repeated measures nested in child; child nested in classroom) was used to disaggregate the observed variation in EF and SRT into between-classroom, between-person, and within-person components. EF composite scores were regressed on two indicators of SRT, which reflected between- and within-child sources of variation, along with demographic covariates (child age, gender, and parental education). Both between-person (b = −21.2, p < 0.001) and within-person (b = −13.2, p < 0.001) sources of SRT variation were uniquely related to EF performance. These results are discussed with respect to interest in using SRT as a proxy for foundational cognitive processes that contribute to EF task performance in early childhood, including the appropriateness of using SRT to refine EF task scores. (author abstract)