Background: Although difficulties in emotion regulation (ER) are considered a core feature of borderline personality disorder (BPD), the specific strategies that individuals with BPD most commonly use, their diagnostic specificity, and their associations with harmful behaviors have not been firmly established. Sampling and Methods: Individuals with BPD (n = 30), mixed anxiety and/or depressive disorders (MAD; n = 30), and healthy controls (HC; n = 32) completed questionnaires assessing both cognitive ER strategies (e.g., cognitive reappraisal) and potentially harmful behaviors that individuals might use to regulate their emotions (e.g., self-injury). Results: BPD subjects endorsed more maladaptive cognitive ER strategies and fewer adaptive strategies compared to HC. Compared to MAD subjects, BPD individuals endorsed more maladaptive cognitive ER strategies, but only when those with subthreshold symptoms in the MAD group were excluded. BPD also endorsed engaging in potentially harmful behaviors more often than both HC and MAD. Discriminant analysis revealed that MAD endorsed lower rates of problem-solving and cognitive reappraisal compared to both HC and BPD. Higher maladaptive and lower adaptive ER strategies were associated with higher rates of potentially harmful behaviors, although the specific strategies differed for MAD versus BPD. Conclusions: BPD and MAD endorse cognitive ER strategies with a comparable frequency, although BPD subjects engage in potentially harmful behaviors more often. Subthreshold BPD symptoms may also affect rates of ER strategy use in individuals with other mental disorders.