Chronic kidney disease (CKD) before treatment and renal function decline during treatment are common in elderly patients receiving hypomethylating agents (HMAs) and venetoclax (VEN). This retrospective multicenter study of 130 newly diagnosed older acute myeloid leukemia (AML) patients evaluated the prognostic impact of renal function before and during the first treatment cycle. A total of 56 patients (43%) had CKD, and 49 (38%) developed acute kidney injury (AKI) during treatment. AKI occurrence was associated with laboratory tumor lysis syndrome (LTLS). CKD before treatment showed a trend towards a lower overall response rate (ORR, OR 0.5, p = 0.07) in multivariable analysis (MVA) and inferior relapse-free survival (RFS, HR 2.16, p = 0.06) in univariate analysis (UVA), but not MVA (RFS, HR 1.93, p = 0.15). CKD did not affect overall survival (OS) or event-free survival (EFS). AKI during HMA/VEN therapy was associated with significantly higher 30-day and 60-day mortality rates and emerged as an independent prognostic factor for inferior OS (HR 1.86, p = 0.01) and EFS (HR 1.81, p = 0.007). RFS did not differ significantly by AKI status. Sepsis was a more frequent cause of death in patients with vs. without AKI (33% vs. 5%). In conclusion, kidney function is a key prognostic factor in HMA/VEN-treated patients, warranting further study on treatment adjustments and supportive care.