ABSTRACT Background Cognitive flexibility is a key factor underlying emotion regulation (ER) and mental health. However, the impact of cognitive flexibility on flexible deployment of ER strategies in changing contexts remains unknown. This study investigated the effects of cognitive flexibility on two noteworthy ER constructs (strategy use and flexibility) and examined downstream impacts on negative affect. Methods Healthy adult participants (N = 202) completed a 10-day experience sampling protocol. Cognitive flexibility, daily ER (including flexibility and ten specific strategies) and negative affect in daily life were measured. We conducted multilevel regression and mediation models to examine associations among cognitive flexibility, daily ER, and negative affect in daily life. Results Higher cognitive flexibility predicted higher ER flexibility – indicated by strategy – situation fit, use of meta-ER skills and between-strategy variability – as well as higher use of reappraisal and problem solving, but lower use of worry. Mediation analyses suggested that enhanced ER flexibility and reduced use of worry linked cognitive flexibility to reduced negative affect. Conclusions Overall, findings have important implications for understanding the effects of cognitive flexibility on rigid versus flexible ER in ever-changing situations.