Objective: To assess the long-term efficacy and safety of erenumab in the subgroup of patients with chronic migraine (CM) in whom prior preventive treatments had failed (TF) (≥1, ≥2, and ≥3 TF medication categories) and never failed (preventive naïve or prior preventive treatments had not failed), using the data from a 52-week, open-label treatment period (OLTP) of the parent study. Background: Erenumab is a fully human monoclonal antibody that selectively binds to and inhibits the canonical calcitonin gene-related peptide receptor. There are limited long-term data evaluating the efficacy and safety of erenumab in patients with CM in whom prior preventive treatments had failed. Methods: Patients who had completed the 12-week double-blind treatment period (DBTP) in the parent study were eligible to participate in the 52-week OLTP, during which they received erenumab every 4 weeks. The TF subgroups (≥1, ≥2, and ≥3 TF medication categories) were not mutually exclusive; patients in whom prior preventive treatments from ≥3 medication categories had failed were also counted in the ≥2 and ≥1 medication categories. Endpoints included monthly migraine days (MMD), monthly acute migraine-specific medication days (MSMD), achievement of ≥50%, ≥75%, and 100% reduction from baseline in MMD, and exposure-adjusted patient incidence rates of adverse events (AEs; per 100 patient-years). Results: Erenumab treatment provided sustained mean reductions in MMD and MSMD relative to the parent study baseline throughout the 52 weeks of the OLTP across all TF subgroups. At Week 52, the mean MMD change was −8.6 (SD 6.6) (baseline: 18.4 [SD 4.5] days) in the ≥1 TF subgroup. A post hoc completer analysis (52 weeks [OLTP] erenumab) showed that compared with erenumab 70 mg, the 140 mg dose was associated with numerically greater reductions in the mean MMD (Week 40: −8.6 and −7.2 days; Week 52: −9.7 and −7.9 days [≥1 TF subgroup]) and a higher proportion of patients achieved ≥50%, ≥75%, and 100% response thresholds across all subgroups at Weeks 40 and 52. Overall the exposure-adjusted patient incidence rates of AEs did not increase during the OLTP versus the DBTP (≥1 TF subgroup: 141.9/100 versus 317.9/100 patient-years), and no new safety signals occurred. Conclusion: The long-term treatment with erenumab was well tolerated and showed sustained efficacy in patients with CM in whom prior preventive treatments had failed, with numerically greater treatment effects for 140 mg versus 70 mg.