Self-care is among the emerging types of mental health support which operate outside traditional services, although the meaning and practice of self-care for young people with mental health difficulties are currently unclear. This systematic review was pre-registered with PROSPERO (CRD42021282510) and investigated conceptualizations of self-care in academic publications which investigated or discussed self-care for young people’s mental health or wellbeing. A Patient and Public Involvement (PPI) workshop facilitated young people with experience of mental health difficulties to respond to the identified concepts and co-develop a definition of self-care. Searches in PsycINFO, MEDLINE, Embase, CINAHL Plus, Scopus, Cochrane Library of Systematic Reviews, and gray literature sources resulted in 90 included publications. Content analysis indicated little conceptual consistency, with health and wellness promotion most commonly used to define self-care. The PPI workshop co-developed a definition of mental health self-care, which attendees felt should emphasize an individual process of self-awareness, self-compassion, and specific strategies to work toward emotional balance. This study highlights the gap between current academic understandings of young people’s mental health self-care and young people’s experience. The presented definition will enable future research to begin from an understanding of self-care which is relevant to young people with experience of mental health difficulties.