Background Gastrointestinal perforation peritonitis is a life-threatening surgical emergency with high mortality. Early identification of patients at increased risk of poor outcomes is critical for optimizing care. In this study, we aimed to evaluate the prognostic value of combining serial Acute Physiology and Chronic Health Evaluation II (APACHE II) scores and serial serum lactate levels in predicting 30-day postoperative mortality among patients undergoing emergency laparotomy for gastrointestinal perforation peritonitis. Methods In this prospective cohort study, 120 adult patients diagnosed with gastrointestinal perforation peritonitis and undergoing emergency laparotomy were enrolled after obtaining ethical approval and informed consent. APACHE II scores and serum lactate levels were recorded at three time points: preoperatively (baseline), 6 h postoperatively, and 24 h postoperatively. The primary objective of this study was a combination of serial APACHE II and serial serum lactate level (baseline, 6 h and 24 h post-operatively) and its correlation with post-operative mortality in emergency laparotomy for hollow viscus perforation peritonitis. Data were analysed to compare clinical variables between survivors and non-survivors. Receiver operating characteristic (ROC) curves and area under the curve (AUC) analyses were used to assess the predictive performance of individual and combined markers. Results The 30-day postoperative mortality rate was 35%. Significant differences in age, serial APACHE II scores, and serial serum lactate levels were observed between survivors and non-survivors. A serum lactate level of ≥ 1.88 mmol/L had a sensitivity of 81% and specificity of 69.2% (AUC: 0.817; p = 0.0001). APACHE II scores of ≥ 11.16 yielded a sensitivity of 76.2% and specificity of 91% (AUC: 0.915; p = 0.0001). Both serial lactate levels and APACHE II scores were independent predictors of 30-day mortality. The combination of serial APACHE II scores and serum lactate (cutoff ≥ 11.95) had a sensitivity of 85%, specificity of 82%, and an AUC of 0.919 (p = 0.0001), making it the preferred predictor for 30-day post-operative mortality. Conclusions The combination of serial APACHE II scores and serial serum lactate levels provides superior prognostic accuracy for predicting 30-day postoperative mortality in patients undergoing emergency laparotomy for gastrointestinal perforation peritonitis. This approach may facilitate early identification of high-risk patients and guide clinical decision-making. Trial registration Not applicable.