Introduction Major abdominal surgery is still a great contributor to postoperative morbidity and mortality in developing countries. Major abdominal surgery leads to hypoperfusion, which has an impact on postoperative morbidity and mortality. Lactate, a biomarker for hypoperfusion is under utilized in Uganda. The study aimed to investigate the association between elevated serum lactate and outcomes (in-hospital mortality, SSI and length of hospital stay) in patients following major abdominal surgery. Methods A prospective observational cohort study was done with 246 eligible patients recruited. Stratified sampling was carried out till desired sample size was achieved. Demographic and perioperative data were collected, serum lactate levels were measured at induction and immediately after surgery with serial measurements being done after 12, 24 h post operatively. Participants were followed up to assess outcomes. Data analysis was done using STATA version 14.0. Results A total of 130 patients (52.8%) had elevated serum lactate levels. Elevated serum lactate predicted in-hospital mortality and surgical site infection. The accuracy of elevated serum lactate to predict mortality with AUROC of 0.7898 was exhibited by the 24 h lactate values. Elevated serum lactate predicted surgical site infection accurately with AUROC 0.6432. Length of hospital is strongly associated with elevated serum lactate with p-value of 0.043. Patients with elevated serum lactate on average have a longer length of hospital stay at 5.34 ± 0.69. Conclusion Elevated serum lactate was associated with in-hospital mortality, surgical site infection and longer length of hospital stay. Serum lactate levels done at 24 h were most predictive of mortality and surgical site infection.