Background: Erenumab is a human anti-calcitonin gene-related peptide receptor monoclonal antibody approved for migraine prevention. We sought to further assess the temporal patterns of response to erenumab in patients with chronic migraine (CM), specifically the onset and sustainability of monthly migraine day (MMD) response. Methods: This is a post hoc analysis of a 12-week, randomized, double-blind, placebo-controlled study of erenumab for migraine prevention in patients with CM (≥15 headache days/month, including ≥8 migraine days/month). Onset and sustainability were assessed according to MMD reduction from baseline, with the following response categories: responders (≥50% reduction), partial responders (≥30% and <50%), or nonresponders (<30%). Results: Among the erenumab 140 mg group (n = 187), 54.0% (101/187) achieved a response at any month during the study with a median time to onset of monthly response of 1 month. This improvement was maintained in most patients with continued treatment. An initial response was achieved at Month 1 by 28.3% (53/187) of patients; 69.8% (37/53) of whom maintained a response at Months 2 and 3. Although many patients responded early, some patients required longer treatment to achieve a response; 79.4% (27/34) of initial partial responders and 21.0% (21/100) of initial nonresponders subsequently achieved a response. Similar findings were observed for the erenumab 70mg group (n = 188). Conclusion: A majority of erenumab-treated patients with CM who achieved an initial response at Month 1 sustained this benefit. Many patients responded later with continued treatment. Our data support recommendations to assess outcomes after ≥3 months of preventive treatment with erenumab in CM.