Rationale: Healing after myocardial infarction involves the biphasic accumulation of inflammatory lymphocyte antigen 6C (Ly-6C) ^high and reparative Ly-6C ^low monocytes/macrophages (Mo/MΦ). According to 1 model, Mo/MΦ heterogeneity in the heart originates in the blood and involves the sequential recruitment of distinct monocyte subsets that differentiate to distinct macrophages. Alternatively, heterogeneity may arise in tissue from 1 circulating subset via local macrophage differentiation and polarization. The orphan nuclear hormone receptor, nuclear receptor subfamily 4, group a, member 1 (Nr4a1), is essential to Ly-6C ^low monocyte production but dispensable to Ly-6C ^low macrophage differentiation; dependence on Nr4a1 can thus discriminate between systemic and local origins of macrophage heterogeneity. Objective: This study tested the role of Nr4a1 in myocardial infarction in the context of the 2 Mo/MΦ accumulation scenarios. Methods and Results: We show that Ly-6C ^high monocytes infiltrate the infarcted myocardium and, unlike Ly-6C ^low monocytes, differentiate to cardiac macrophages. In the early, inflammatory phase of acute myocardial ischemic injury, Ly-6C ^high monocytes accrue in response to a brief C–C chemokine ligand 2 burst. In the second, reparative phase, accumulated Ly-6C ^high monocytes give rise to reparative Ly-6C ^low F4/80 ^high macrophages that proliferate locally. In the absence of Nr4a1, Ly-6C ^high monocytes express heightened levels of C–C chemokine receptor 2 on their surface, avidly infiltrate the myocardium, and differentiate to abnormally inflammatory macrophages, which results in defective healing and compromised heart function. Conclusions: Ly-6C ^high monocytes orchestrate both inflammatory and reparative phases during myocardial infarction and depend on Nr4a1 to limit their influx and inflammatory cytokine expression.