Objective: To investigate the role of calcitonin gene-related peptide (CGRP) in persistent post-traumatic headache (PTH) attributed to mild traumatic brain injury (TBI). Methods: A total of 100 individuals with persistent PTH attributed to mild TBI and 100 age- and gender-matched healthy controls were enrolled between July 2018 and June 2019. Blood was drawn from the antecubital vein and subsequently analyzed using a validated radioimmunoassay for human CGRP. Measurements were performed on coded samples by a board-certified laboratory technician who was blind to clinical information. Results: CGRP plasma levels were lower in subjects with persistent PTH (mean, 75.8 pmol/L; SD, 26.4 pmol/L), compared with age- and gender-matched healthy controls (mean, 88.0 pmol/L; SD, 34.1 pmol/L) (p = 0.04). No correlation was found of CGRP plasma levels with monthly headache days (r = −0.11; p = 0.27), monthly migraine-like days (r = 0.15; p = 0.13), headache quality (r = −0.14; p = 0.15), or a chronic migraine-like headache phenotype (r = −0.02; p = 0.85). Conclusions: CGRP plasma measurements are unlikely a feasible blood-based biomarker of persistent PTH. Future studies should assess whether CGRP plasma measurements can be used to predict development of persistent PTH.