High-risk violent offenders often present challenges for rehabilitation, with lower engagement and completion rates. Responsivity—the “how” of the Risk-Need-Responsivity model—remains under-researched, particularly regarding the role of trauma. This study analysed data from 423 offenders in New Zealand’s Special Treatment Units. Childhood maltreatment was pervasive (82% experienced at least one type; many experienced multiple), and the number of maltreatment types significantly predicted poorer treatment progress and higher dropout. PTSD symptoms were not significantly related to outcomes, suggesting enduring effects of early trauma—not just current symptomology—undermine rehabilitation. Findings support integrating trauma-informed approaches into male offender programmes, echoing practices already applied with female populations.