Background: This study evaluated the prognostic impact of AKI duration on 90-day mortality and new-onset chronic kidney disease (CKD) progression in very elderly patients. Methods: We retrospectively enrolled very elderly patients (≥75 years) with normal RF from the hospital information system of the National Clinical Research Center for Geriatric Diseases of Chinese PLA General Hospital January 1, 2007 and December 31, 2018. AKI was diagnosed using the Kidney Disease: Improving Global Outcomes criteria. AKI patients were divided into T-AKI and P-AKI groups based on whether serum creatinine level returned to baseline within 48 h post-AKI. Results: In total, 760 geriatric patients developed AKI, and 693 were suitable for the final evaluation. Among them, 62 (8.9%) patients had T-AKI (1–2 days), 104 (15.0%) had short-duration AKI (3–4 days), 140 (20.2%) had medium-duration AKI (5–7 days), and 387 (55.8%) had long-duration AKI (>7 days). In total, 209 (30.2%) of 693 patients died within 90 days, including 5 (8.1%) with T-AKI and 204 (32.3%) with P-AKI. Of the 484 survivors with AKI, 122 (25.2%) developed CKD. After adjusting for multiple covariates, duration of AKI (3–4 days: hazard ratio [HR] = 2.512; 95% confidence interval [CI]: 1.021–6.181; 5–7 days: HR = 3.154; 95% CI: 1.250–7.960; >7 days: HR = 6.212; 95% CI: 2.383–16.192;), more advanced AKI stages (stage 2: HR = 7.365; 95% CI: 4.114–13.183; stage 3: HR = 28.414; 95% CI: 16.360–49.350); and low body mass index (HR = 0.910; 95% CI: 0.870–0.953) were significantly associated with a higher 90-day mortality and longer AKI duration (3–4 days: odds ratio [OR] = 0.982; 95% CI: 0.247–3.900; 5–7 days: HR = 1.322; 95% CI: 0.381–4.592; > 7days: HR = 7.007; 95% CI: 2.417–20.311), and baseline eGFR (OR = 0.928; 95% CI: 0.901–0.956) was significantly associated with new-onset CKD of the survivors. Conclusion: AKI duration is a useful parameter to predict of worse clinical outcomes in very elderly patients, emphasizing the importance of identifying an appropriate treatment window for early intervention.