Uncontrollable anger is a debilitating consequence of acquired brain injury (ABI). This proof-of-concept study investigated the preliminary efficacy of an emotion regulation intervention for managing post-ABI anger. A secondary objective was to determine which participant characteristics were related to intervention gains. With a pre-post intervention design and three-month follow-up, there were five individually administered meetings on Zoom, over a four-month period. 24 adults who had sustained an ABI were enrolled. Participants were mostly males, from 24 to 85 years old. A series of one-way repeated-measures ANOVAs were conducted to determine the intervention’s efficacy, and Spearman’s rho bivariate correlations for the association between participant characteristics and intervention gains. Significant differences were observed in external anger from baseline to post-treatment; there were no further changes from post-treatment to follow-up. Of the participant characteristics, only readiness to change and anxiety were correlated. The proposed intervention presents a brief, feasible, and preliminary efficacious alternative for regulating post-ABI anger. Intervention gains are associated with readiness to change and anxiety, which has important implications for clinical delivery.