Past research in developmental psychology has revealed close links between affect and planning ability over the life-span. However, such links have rarely been studied in children from non-clinical samples. To address this gap, the present study investigates whether negative affect, understood as a temperamental trait, affects planning ability in 5-year-old children. Moreover, it examines whether selected cognitive processes, that is inhibition, verbal working memory, and cognitive flexibility, mediate this relationship. A total of 103 children aged M = 5 years 4 months, SD = 2.7 months, and their parents participated in the study. The analyses, which employed a path mediation model, revealed that negative affect's influence on planning was entirely mediated by flexibility. Moreover, flexibility partially mediated the relationship between inhibition and planning, and verbal working memory had a direct effect on planning. The results provide a new insight into the relationship between negative affect and planning ability in non-clinical preschool children.