Patients suffering from mental illness are vulnerable, and they do not always have access to proper emergency care. The aim of this study was to identify competency requirements for the assessment of patients with mental illness by soliciting the views of emergency care nurses. A modified Delphi method comprising four rounds was used. Data were collected in Sweden between October 2018 and March 2019. The data were analyzed using content analysis and descriptive statistics. The panel of experts reached the highest level of consensus regarding basic medical knowledge: the capability to listen and show respect to the patient are essential competency requirements when assessing patients with mental illness in emergency care. Awareness of these competency requirements will enhance teaching and training of emergency care nurses.