A substantial body of research has examined emotion regulation (ER) deficits in connection with borderline personality disorder (BPD) based on individuals’ use of specific ER strategies. However, studies historically have focused on the putative adaptability or maladaptability of individual strategies without adequate focus on the fit between specific strategies and context. Thus, ER flexibility as an index of the ability to vary the use of multiple ER strategies based on changing environmental demands and goals may allow for such an approach. Yet, to date, there remains uncertainty about how to best operationalize ER flexibility, and little research has examined this construct in relation to BPD. Further, the requisite processes presumed to be involved in ER flexibility appear to involve higher level cognitive skills. Therefore, the present study sought to develop models of ER flexibility to examine the proposed associations between (a) higher BPD traits and decreased ER flexibility, (b) greater ER flexibility and better executive functioning, and to (c) examine the potential mediating role of executive functioning between ER flexibility and BPD traits. Participants (N = 250) in the present study were recruited via the Mturk platform and completed a novel behavioral paradigm and 2 weeks of daily diary measures of emotion, ER, and executive functioning. Findings corroborate existing work showing that greater BPD traits are associated with more executive dysfunction and add to the field of ER flexibility by illuminating how differences in the ways in which ER flexibility is conceptualized and measured can affect associations between dispositional traits, such as BPD, and ER strategy use. Theoretical and clinical implications are also discussed.