Delirium affects 15% of older adults presenting to emergency departments (EDs) but is detected in only one-third of cases. Evidence-based guidelines for ED delirium screening exist, but are underutilized. Frontline staff perceptions about delirium and time and resource constraints are known barriers to ED delirium screening uptake. Early adopters of ED delirium screening can offer valuable lessons about successful implementation. We conducted semi-structured interviews with clinician-administrators leading ED delirium screening initiatives from 20 EDs in the United States and Canada. Interviews focused on experiences of planning and implementing ED delirium screening. Interviews lasted 15 to 50 minutes and were digitally recorded and transcribed. To identify factors that commonly impacted implementation of ED delirium screening, we used constructs from the Consolidated Framework for Implementation Research (CFIR), an Implementation Science framework widely used to evaluate healthcare improvement initiatives. Overall, notable facilitators of successful implementation were having institutional and ED leadership support and designated clinical champions to longitudinally engage and educate frontline staff. We found specific examples of factors affecting implementation drawn from the following seven CFIR constructs: (1) intervention complexity, (2) intervention adaptability, (3) external policies and incentives, (4) peer pressure from other institutions, (5) the implementation climate of the ED, (6) staff knowledge and beliefs, and (7) engaging deliverers of intervention, that is, frontline ED staff. Implementing ED delirium screening is complex and requires institutional resources as well as clinical champions to engage frontline staff in a sustained fashion.