For patients with mental illnesses (MIs), emergency departments (EDs) are often the entry point into the healthcare system, or their only resort for quickly accessing mental health treatment. A better understanding of the various barriers justifying high ED use among patients with MIs may help recommend targeted interventions that better meet their needs. This explorative qualitative study aimed to identify such barriers and the solutions brought forth to reduce ED use based on the perspectives of clinicians and managers working in EDs, other hospital departments or the community sector. Interviews were conducted between April 2021 and February 2022; 86 mental health professionals (22% were nurses) from four large urban ED sites in Quebec (Canada) were interviewed. Barriers were identified in relation to patient profiles, healthcare system and organisational features and professional characteristics. The key barriers that were found to explain high ED use were patients having serious MIs (e.g., psychotic disorders) or social issues (e.g., poverty), lack of coordination and patient referrals between EDs and other health services, insufficient access to mental health and addiction services and inadequacy of care. Very few solutions were implemented to improve care for high ED users. Better deployment of ED interventions in collaboration with outpatient care may be prioritised to reduce high ED use for patients with MIs. Improvements to the referral and transfer processes to outpatient care, particularly through care plans and case management programs, may be implemented to reduce high ED use and improve outpatient care among patients with multiple health and social needs.