Abstract Purpose: In MONARCH 2, abemaciclib plus fulvestrant significantly prolonged progression-free survival (PFS) and overall survival (OS) versus placebo plus fulvestrant in patients with hormone receptor positive (HR+), HER2− advanced breast cancer. This exploratory analysis assessed the efficacy of abemaciclib plus fulvestrant across subgroups of patients receiving study therapy as first- or second-line treatment for metastatic disease. Patients and Methods: Improvements were estimated using Cox models, and a test of interactions of subgroups with treatment was performed. Results: The benefit in PFS [first-line, HR, 0.57; 95% confidence interval (CI), 0.45–0.73; second-line, HR, 0.48; 95% CI, 0.36–0.64] and OS (first-line, HR, 0.85; 95% CI, 0.64–1.14; second-line, HR, 0.66; 95% CI, 0.46–0.94) was observed across both subgroups, consistent with the intent-to-treat (ITT) population. In first-line patients (abemaciclib arm, n = 265; placebo arm, n = 133), the numerically largest effect on PFS and OS was observed in patients with primary resistance to endocrine therapy (ET; PFS, HR, 0.40; 95% CI, 0.26–0.63; OS, HR, 0.58; 95% CI, 0.35–0.97) and visceral disease (PFS, HR, 0.54; 95% CI, 0.39–0.73; OS, HR, 0.82; 95% CI, 0.58–1.20). In second-line patients (abemaciclib arm, n = 170; placebo arm, n = 86), a numerical benefit in PFS and OS was observed across primary and secondary ET resistance, with numerically more pronounced effects observed in patients with visceral disease (PFS, HR, 0.39; 95% CI, 0.27–0.57; OS, HR, 0.51; 95% CI, 0.33–0.81). Prolongation of time to second disease progression, time to chemotherapy, and chemotherapy-free survival was observed in both subgroups. Conclusions: Consistent with the ITT population, a benefit in PFS and OS was observed across the first- and second-line subgroups in MONARCH 2.