Abstract Background Monoclonal antibodies targeting the calcitonin gene-related peptide (CGRP) pathway, including the fully humanized monoclonal antibody (IgG2Δa) fremanezumab, have demonstrated safety and efficacy for migraine prevention. Clinical trials include responders and nonresponders; efficacy outcomes describe mean values across both groups and thus provide little insight into the clinical benefit in responders. Clinicians and their patients want to understand the extent of clinical improvement in patients who respond. This post hoc analysis of fremanezumab treatment attempts to answer this question: what is the benefit in subjects who responded to treatment during the two, phase 3 HALO clinical trials? Methods We included subjects with episodic migraine (EM) or chronic migraine (CM) who received fremanezumab quarterly (675 mg/placebo/placebo) or monthly (EM: 225 mg/225 mg/225 mg; CM: 675 mg/225 mg/225 mg) during the 12-week randomized, double-blind, placebo-controlled HALO EM and HALO CM clinical trials. EM and CM responders were defined as participants with a reduction of ≥ 2 or ≥ 4 monthly migraine days, respectively. Treatment benefits evaluated included reductions in monthly migraine days, acute headache medication use, and headache-related disability, and changes in health-related quality of life (HRQoL). Results Overall, 857 participants from the HALO trials were identified as responders (EM: 429 [73.8%]; CM: 428 [56.7%]). Reductions in the monthly average number of migraine days were greater among EM (quarterly: 5.4 days; monthly: 5.5 days) and CM (quarterly: 8.7 days; monthly: 9.1 days) responders compared with the overall population. The proportion of participants achieving ≥ 50% reduction in the average monthly number of migraine days was also greater in responders (EM: quarterly, 59.8%; monthly, 63.7%; CM: quarterly, 52.8%; monthly, 59.0%) than in the overall population. Greater reductions in the average number of days of acute headache medication use, greater reductions in headache-related disability scores, and larger improvements in HRQoL were observed among EM and CM responders compared with the overall populations. Conclusions Fremanezumab responders achieved clinically meaningful improvements in all outcomes. The magnitude of improvements with fremanezumab across efficacy outcomes was far greater in responders than in the overall trial population, providing insight into expected treatment benefits in participants who respond to fremanezumab in clinical practice. Trial registration ClinicalTrials.gov identifiers: NCT02629861 (HALO EM) and NCT02621931 (HALO CM).