Studies have established that diabetic patients with community-acquired pneumonia (CAP) may have increased mortality. The primary objective of this study was to investigate if time to first appropriate antibiotic in the emergency department (ED) was associated with in-hospital mortality of CAP in patients with diabetes mellitus (DM). This was a retrospective cohort study of adult diabetic patients who were admitted with CAP. Patients were stratified into two groups: those who received first dose of appropriate antibiotic within 8 hours of triage and those who received it later than 8 hours. A multiple logistic regression analysis was performed. Two hundred six patients were included in the study. Fifty-nine patients (28.6%) had complications of CAP on admission and 31 patients (16%) died. In-hospital mortality was higher in patients who received their initial appropriate antibiotic after 8 hours of triage than those who received it within 8 hours [18 (35.3%), 15 (9.7%), p < 0.0001]. Time to first appropriate antibiotic later than 8 hours of triage was associated with increased in-hospital mortality (OR 4, 95% CI 1.2-13.1, p = 0.02). Antibiotic administration later than 8 hours of triage in the ED was associated with increased in-hospital mortality of CAP among patients with DM.