Background: People living with mental illness form a significant component of individuals presenting to emergency care services. Ambulance care embraces the care and treatment given to people of all ages who have suffered a sudden illness or injury and is carried out twenty-four-seven, regardless of setting and organizational belonging. Aim: The aim was to explore ambulance clinicians’ competence requirements in caring for people living with mental illness. Method: The study had a deductive and explorative design. A Delphi method was adopted using a group of experienced individuals recruited from the emergency care chain and non-governmental organizations (N = 15). An initial open-ended questionnaire was distributed covering three questions about; (1) knowledge, (2) skills and (3) attitudes that ambulance clinicians need to care for people living with mental illness. The informants’ answers were analysed using a manifest content analysis ending up in statements designed into a questionnaire that was sent out digitally in two rounds. Results: The 57 statements that reached consensus could be categorised as referring to knowledge (n = 26), skills (n = 13) and attitude (n = 18). Conclusion: Ambulance clinicians are expected to manage a range of incidents involving people living with mental illness, demanding knowledge of mental illness and the skills of mental health assessment, to ensure ambulance clinicians have the ability and non-judgmental attitude to make appropriate decisions within a caring encounter.