Deficits in divided attention occur after a mild traumatic brain injury (MTBI) but many extant tasks lack sensitivity for detecting subtle cognitive difficulties. We use the Test d'Attention Partage Informatis (TAPI), a novel dual-task paradigm, to investigate the impact of MTBI on the ability to divide attention between two stimuli sources. Individuals with MTBI (n = 37) were evaluated within the first week following head trauma and at three months post-injury. A healthy control (HC) group (n = 79) was also assessed. The primary outcome was reaction time and there were three different conditions that included visual target detection and auditory digit span tasks. Analyses utilised repeated measures ANOVA and ANCOVA models that adjusted for relevant variables including post-concussive and affective symptoms. Results indicated that at both baseline and follow-up, the MTBI group had significantly slower reaction time than the HC group. Also, both the MTBI and HC groups had slower reaction times as participants progressed through each of the more challenging TAPI conditions. This study supports the usefulness of this novel instrument and allows clinicians and researchers to assess for subtle divided attention deficits that may persist in those with MTBI even three months post-injury.