Mental health problems affect people with intellectual disabilities (ID) at rates similar to or in excess of the non-ID population. People with severe ID are likely to have persistent mental health problems. In this systematic review (PROSPERO 2015:CRD42015024469), we identify and evaluate the methodological quality of available measures of mental health problems or well-being in individuals with severe or profound ID. Electronic searches of ten databases identified relevant publications. Two reviewers independently reviewed titles and abstracts of retrieved records (n = 41,232) and full-text articles (n = 573). Data were extracted and the quality of included papers was appraised. Thirty-two papers reporting on 12 measures were included. Nine measures addressed a broad spectrum of mental health problems, and were largely observational. One physiological measure of well-being was included. The Aberrant Behavior Checklist, Diagnostic Assessment for the Severely Handicapped Scale-II and Mood, Interest and Pleasure Questionnaire are reliable measures in this population. However, the psychometric properties of six other measures were only considered within a single study – indicating a lack of research replication. Few mental health measures are available for people with severe or profound ID, particularly lacking are tools measuring well-being. Assessment methods that do not rely on proxy reports should be explored further.