Objectives To explore the cross-sectional association between adiposity, mental well-being, and quality of life in extreme obese individuals entering a UK specialist weight management service prior to treatment commencement. Methods The sample comprised 263 extreme obese individuals who were referred to the service as a result of having a body mass index (BMI) ≥40 kg/m2 or ≥35 kg/m2 with a co-morbid health condition. In a retrospective analysis, routinely collected baseline clinical examination data and self-report questionnaires (Impact of Weight on Quality of Life: IWQOL-Lite, EQ5D-3L, and Hospital Anxiety and Depression Scale: HADS) were analysed to examine the cross-sectional association between adiposity and quality of life. Results The sample was predominantly female (74.8%) with mean BMI 47.0±7.9 kg/m2. Increasing adiposity was significantly negatively associated with quality of life, with an increase of 1 BMI unit associated with decreases of 1.93 in physical function (95% CI −2.86 − −1.00, p<0.001), 1.62 in self-esteem (95% CI −2.67 − −0.57, p<0.05), 2.69 in public distress (95% CI −3.75 − −1.62, p<0.001), 1.33 in work (95% CI −2.63 − −0.02, p<0.05), and 1.79 in total IWQOL-Lite scores (95% CI −2.65 − −0.93, p<0.001). Adiposity was associated with significantly increased risk of problems in mobility (OR = 3.44, 95% CI 1.47−8.05), and performing usual activities (OR = 2.45, 95% CI 1.10−5.46) in highest relative to lowest BMI tertile. The prevalence of experience of symptoms of anxiety (70.3%) and depression (66.2%) as measured by HADS was consistently high. Conclusions We identified a high prevalence of psychological co-morbidity, including widespread experience of symptoms of anxiety and depressive disorders and reduced quality of life among these extreme obese individuals seeking weight management treatment. Clinical implications include the need for the incorporation of strategies to improve mental well-being into multi-disciplinary weight management interventions.