Background: An individual’s understanding of mental health can influence their attitudes towards those experiencing mental health problems, and also impact their response to any mental health problems they experience. However, what the lay public understand about mental health is not well explored in existing research. Aims: This study aims to gain a deeper insight into what the general public understand by the term ‘mental health problem’. Methods: Data were taken from a large-scale representative sample of adults from Great Britain ( n = 2,708). A thematic analysis was carried out on an open-text question which asked people what they understood by the term ‘mental health problem’. Results: Six themes were identified in the thematic analysis, which included understanding mental health through thinking about cause and effect, thinking about the location of mental health problems in the body, the universality and variation of mental health problems, reflections on lived experience and identifying a specific mental health problem. Conclusion: The analysis suggests that there are many diverse ways the public conceptualises mental health. The themes identified may be useful for future quantitative analyses, and also may suggest how information about mental health can be best communicated to the public.