Objectives: Kidney dysfunction in cirrhosis is strongly associated with adverse short-term complications; however, its long-term prognosis and clinical predictors remain underinvestigated. This study aimed to evaluate the long-term outcomes of rapid kidney function decline—defined as a reduction of >5 mL/min/1.73 m² per year—and to identify its clinical determinants in patients with compensated cirrhosis. Methods: We conducted a retrospective cohort study of patients with Child–Pugh class A cirrhosis who attended Thammasat University Hospital between 2016 and 2025. Patients with Child–Pugh class B or C cirrhosis were excluded. The primary outcome was all-cause mortality, analyzed using Cox proportional hazards models. Secondary outcomes included hospitalization, hepatic encephalopathy (HE), spontaneous bacterial peritonitis (SBP), sepsis, and heart failure (HF). Clinical predictors of rapid kidney function decline were assessed using multivariable logistic regression. The median follow-up duration was 8.4 years. Results: Among 329 patients (mean age 62.6 years; 60% male), 71 (21.5%) experienced rapid kidney function decline. These patients had a markedly higher risk of all-cause mortality (adjusted hazard ratio [HR] 9.25; 95% CI 4.62–18.51; p < 0.001), as well as higher risks of sepsis-related hospitalization (adjusted HR 3.65; 95% CI 1.75–7.62) and hepatic encephalopathy (adjusted HR 13.08; 95% CI 2.03–84.04). Multivariable logistic regression identified ascites (adjusted OR 5.73), diabetes mellitus (adjusted OR 3.30), furosemide use (adjusted OR 3.37), and prior hospitalization (adjusted OR 2.46) as independent clinical predictors of rapid kidney function decline. Conclusion: Rapid kidney function decline is a strong predictor of mortality in patients with compensated cirrhosis. Ascites, diabetes mellitus, furosemide use, and prior hospitalization are key clinical predictors.