While mental disorders have been broadly researched in people with intellectual disabilities (ID), comparatively less attention has been given to the conceptualisation of good mental health for this population. To capture existing concepts, definitions and measurement approaches of good mental health a systematic literature review was conducted following PRISMA guidelines. The search was carried out in eleven databases, using various synonyms of (i) intellectual disability, (ii) mental health, (iii) wellbeing, (iv) definition, and (v) assessment. A total of 2,046 datasets were identified, of which 37 met the inclusion criteria and were analysed using reflexive thematic analysis and content analysis. Results show four main themes: (1) environment, (2) absence of mental illness, (3) physical health, and (4) psychosocial functioning. The fourth was the most dominant theme and was further divided into five sub-themes: (1) emotionality, (2) interpersonal relations, (3) realise own potential, (4) personal resources, and (5) overall appraisal of life. Our findings reveal different conceptualisations of wellbeing, which is a vital part of good mental health, but highlight a notable research gap in the actual definition and conceptualisation of good mental health for people with ID.