Primary decompressive craniectomy (DC) is performed alongside the removal of mass lesions from traumatic brain injury (TBI). The Helsinki computed tomography (CT) score predicts post-TBI outcomes and is partly based on the size of the mass lesion. It remains unclear if this scoring system can be applied to this subgroup and exhibit discriminative ability. This study sought to validate the prognostic value of the Helsinki CT score in TBI patients undergoing primary DC. In this retrospective analysis, we evaluated 187 patients who underwent primary DC for TBI. Short-term outcomes, identified at discharge, included all participants (N = 187), while long-term outcomes were ascertained either through a minimum 12-month follow-up or up to the patient's death (N = 162). The Glasgow Outcome Scale (GOS) score of 1-3 denoted unfavorable outcomes. Higher Helsinki CT scores corresponded with an increase in unfavorable outcomes. The Helsinki CT score's predictive capability for these outcomes was evident in both the short term (OR: 1.36, 95% CI: 1.08-1.71, p = 0.009) and long term (OR: 1.30, 95% CI: 1.01-1.67, p = 0.041). The area under the Receiver Operating Characteristic curves was 0.743 (p <
0.001) for the short term and 0.686 (p <
0.001) for the long term. In conclusions, the Helsinki CT score holds considerable prognostic value following primary DC for TBI. Our findings suggest that the size of the mass lesion is not the predominant factor in prognostic judgments after primary DC, nor does it reduce the Helsinki score's utility.