Background: Dedicated geriatric models of care are becoming more prevalent due to the complexity of, and increase in, acute healthcare presentations for older patients. For older people, a long stay in the emergency department (ED) may reflect the complexity of their presentation, or deficiencies in systems that manage these complexities. Aims: To identify predictors of a long ED length of stay (LLoS) for patients ≥65 years old. Methods: Linked hospital information systems data from a large, public Australian ED were analysed in this retrospective cohort study. LLoS was defined as the 75th percentile (617 min). Multivariate regression identified LLoS predictors for admissions and discharges separately. Results: Of 16 791 ED presentations made by older people, 4192 experienced a LLoS; 55% were admitted. Increasing age was associated with an increasing ED LoS. Factors most predictive of LLoS for both admitted and discharged patients included: investigations (both pathology and imaging), less urgent Australasian triage scale categories and after-hours arrival. Ambulance arrival did not increase the risk of a LLoS for patients eventually admitted, but conferred nearly a twofold increased risk for a LLoS for discharged older persons (adjusted odds ratios = 1.9; 95% confidence interval 1.5–2.4). Conclusions: This study assists clinicians and decision-makers to identify reasons why older persons have a LLoS, whether admitted or discharged. Interventions to streamline care for older patients arriving after-hours and who require imaging and pathology are required. LoS targets should consider age distribution. The use of ED LoS as a quality of care indicator should be assessed for admissions and discharges, separately.