Introduction: National Early Warning Score 2 (NEWS2) is widely used to monitor and trigger assessment throughout a patient's hospital journey. Since the development and role out of NEWS2, its ability to predict mortality has been assessed in several settings, although to date not within an undifferentiated ED population. Methods: We conducted a retrospective observational study of all adult ED attendees at two EDs in Northern England, between March and November 2019. Multilevel multiple logistic regression analyses were conducted on patient episode data to assess the relationship between mortality at 2, 7 and 30 days from attendances; and maximum NEWS2, adjusting for age, sex, arrival mode and triage priority. Results: Data were collected from 91 871 valid patient episodes associated with 64 760 patients. NEWS2 was a significant predictor of mortality at 2 days (OR 1.75; 95% CI 1.58 to 1.93); at 7 days (OR 1.69; 95% CI 1.59 to 1.80); at 30 days (OR 1.58; 95% CI 1.52 to 1.64). For the analyses of categorised NEWS2, NEWS2 of 2-20 was significantly associated with mortality at 2, 7 and 30 days compared with none assigned: OR 3.54 (95% CI 2.15 to 5.85) at 2 days; OR 6.05 (95% CI 3.92 to 9.34) at 7 days; OR 12.4 (95% CI 7.91 to 19.3) at 30 days. Increasing age, male sex, arrival by ambulance and higher triage categories were also associated with significantly increased mortality. Area under the receiver operating characteristic curve values of 0.963, 0.946 and 0.915, respectively, were recorded for mortality outcomes, with optimum likelihood ratios associated with a trigger of 4 NEWS2 points. Conclusions: NEWS2 is an effective predictor of mortality for patients presenting to the ED. Findings suggest that maximum NEWS2 of 4 and over may be the best trigger point for escalation of treatment. Findings also suggest a NEWS2 of 0-1 can identify a very low-risk group within the ED.