Background: Patients with ST-segment elevation myocardial infarction (STEMI) and primary percutaneous coronary intervention (PPCI) on a left main culprit lesion have very high mortality rates. The interaction of chronic kidney disease (CKD) with such a catastrophic acute event on the background of their highly complex atherosclerotic lesions is not well established. Therefore, we sought to evaluate in these patients the influence of the estimated glomerular filtration rate (eGFR) on short- and long-term mortality. Methods: We retrospectively analyzed renal function in 81 patients with STEMI and PPCI on a left main culprit lesion from two tertiary centers. Results: Patients were divided in two groups according to an eGFR cut-off of 60 mL/min/1.73 m2: 40 patients with CKD and 41 without CKD. Patients with renal failure were older, had more diabetes, and had experienced more frequent myocardial infarction MIs. CKD patients had a higher baseline-SYNTAX score (p = 0.015), higher residual-SYNTAX score (p < 0.001), and lower SYNTAX revascularization index-SRI (p = 0.003). Mortality at 30-day, 1-year, and 3-year follow-ups were not significantly different between the two groups. However, when analyzed as a continuous variable, eGFR emerged as a predictor of 1-year mortality, both in univariate analysis (OR = 0.97, 95% CI: 0.95–0.99, p = 0.005) and in multivariate analysis, after adjusting for cardiogenic shock and Thrombolysis in Myocardial Infarction TIMI 0/1 flow (OR = 0.975, 95% CI: 0.95–0.99, p = 0.021). Conclusions: In STEMI with PPCI on a left main culprit lesion, renal failure was associated with more complex coronary lesions and less complete revascularization, and turned out to be an independent predictor of mortality at 1-year follow-up.