This study aimed to describe characteristics, outcomes, and post-emergency department (ED) departure medical requirements of patients who did not wait (DNW) or left against medical advice (LAMA) after presenting to an Australian hospital ED over a 6-month period. This was a prospective cohort follow-up study. Children and adults were compared in terms of ED characteristics and outcomes. Of the 32,333 patient presentations, 3,293 (10.2%) were recorded as DNW and 470 (1.5%) as LAMA. Of the DNW/LAMA presentations, 1,303 (34.6%) received a telephone call. One in four of the DNW/LAMA patients were children (<16 years). Most (87%) waited longer than the recommended time before leaving the ED, the majority (56%) sought care elsewhere, and some (n = 174, 13%) re-presented to the ED within 7 days; 20 of those required hospital admission. Strategies addressing front-end ED systems are required to mitigate the proportion of patients who DNW/LAMA.