Introduction Limited studies are available for predicting mortality in patients with spontaneous bacterial peritonitis (SBP) based on ascitic fluid analysis. Recently, a proposition has been made regarding the role of ascitic fluid lactate as a better prognostic indicator of mortality in cirrhotic patients with SBP. Therefore, we aimed to evaluate the utility of ascitic fluid lactate in predicting mortality in cirrhotic patients with SBP. Methods This was a prospective, observational study that was conducted in the Hepato-Gastroenterology Department of Sindh Institute of Urology and Transplantation (SIUT), Karachi from 1 January 2022 to 31 December 2022. All the patients having liver cirrhosis with ascites, aged between 18 and 65 years, and presenting with fever and/or abdominal pain were recruited in the study in the first six months (i.e., from 1 January 2022 to 30 June 2022) and were followed for six more months for the outcome. However, those patients on dialysis or those with hepatocellular carcinoma, any other malignancy as per a history of solid organ transplant, a history of HIV infection, or those underlying systemic sepsis or infections other than SBP were excluded from the study. The presence or absence of SBP was confirmed by doing the ascitic fluid analysis. Ascitic fluid lactate levels were also requested in each patient. Mortality was assessed at one, two, three, and six months, respectively. All the data were analyzed using SPSS version 23.0. The area under the receiver operating curve (AUROC) was obtained for ascitic fluid lactate for predicting mortality in SBP. At an optimal cutoff, the diagnostic accuracy of ascitic fluid lactate was obtained. Results The total number of cirrhotic patients included in the study was 123. The majority of the patients belong to Child Turcotte Pugh (CTP) class C (n = 88; 71%). Two third of the patients (65.8%; n = 81) had viral hepatitis i.e., hepatitis B, D, and/or C, as the cause of cirrhosis. Overall mortality was observed in 51(41.5%) patients. Ascitic fluid lactate was significantly raised in patients with SBP than in patients with non-SBP (p = 0.004). The AUROC of ascitic fluid lactate was highest at three months (AUROC = 0.88) followed by six months (AUROC = 0.84), two months (AUROC = 0.804), and one month (AUROC=0.773). At an optimal cut-off of more than or equal to 22.4 mg/dl, ascitic fluid lactate had a sensitivity of 84.9%, specificity of 85.7%, positive predictive value (PPV) of 97.3%, negative predictive value of 42.8% with diagnostic accuracy of 85% in predicting overall mortality in patients with SBP. On sub-analysis, the diagnostic accuracy of ascitic fluid lactate was highest at six months followed by at three, two, and one month, respectively. Conclusion Ascitic fluid lactate showed a good diagnostic utility in predicting the overall mortality in patients with SBP with the best diagnostic accuracy in predicting long-term (six months) mortality. However, further studies are required to validate our results.