Abstract Objective To evaluate the utility of the AM-PAC (Activity Measure for Post-Acute Care) for functional assessment of patients with Traumatic Brain Injury (TBI) across inpatient and outpatient rehabilitation settings. We evaluated (1) Longitudinal changes (1-year) in cognitive and physical recovery patterns across the rehabilitation continuum (2) Concurrent validity of AM-PAC as compared to traditional measures. Method A prospective repeated measures cohort study of adults with TBI (n = 186), where 87 participants received inpatient rehabilitation (IR), and 99 participants received outpatient cognitive rehabilitation (CR). Functional recovery was assessed at IR admission, discharge, 6 months and 1 year, and at admission/discharge from CR for post-acute participants. Functional measures included AM-PAC-Basic Mobility (BM), Daily Activities (DA), and Applied Cognitive (AC) domains, Neuropsychological Assessment Battery (NAB), Texas Functional Living Scale (TFLS), Functional Independence Measure (FIM), Participation Assessment with Recombined Tools-Objective (PART-O), and Mayo-Portland Adaptability Inventory (MPAI-4: self-awareness). Results The AMPAC was sensitive to changes throughout the care continuum, demonstrating medium effect sizes across all three functional domains. Notably, AMPAC scores showed greater change compared to traditional tools (90% and 68% confidence levels). Additionally, analysis of Minimum Detectable Change thresholds demonstrated significant improvements, across settings and within moderate/severe TBI (p < 0.05), followed by predictably smaller gains in the mild TBI population (p < 0.05), underscoring the clinical relevance of these findings. Conclusions Results demonstrate that the AM-PAC is a practical, valid and sensitive scale responsive to longitudinal ecological and functional change in TBI. Analysis confirms the effectiveness of functional assessments, particularly emphasizing strengths of the AMPAC.