The current study tests whether accuracy and reaction time (RT) on the Hearts and Flowers (HF) task, a common assessment tool used across wide age ranges, can be leveraged as joint indicators of child executive function (EF) ability. Although previous studies have tended to use accuracy or RT, either alone or as separate indicators, one open question is whether these 2 metrics can be yoked together to enhance our measurement of EF ability. We test this question using HF data collected from first-grade children who participated in the Family Life Project. Specifically, we model the independent and interactive effects of HF accuracy and RT on several criterion outcomes representing child academic and behavioral competence. Our findings indicate that among early-elementary-aged children, accuracy and RT interact in the prediction of child outcomes, with RT being a more informative index of EF ability for children who perform at high levels of accuracy. The main effect of accuracy remained significant in the presence of these interactive effects. This pattern of findings was similar for different task blocks (i.e., mixed, flower-only) and for different child outcome domains (i.e., academic, behavioral). Our finding of an interaction between accuracy and RT contributes to a growing literature that attempts to jointly consider accuracy and RT as indicators of underlying ability, which has important implications for how EF task scores are constructed and interpreted. (PsycINFO Database Record (c) 2019 APA, all rights reserved).