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Objective Although body mass index (BMI)-defined non-obesity metabolic dysfunction-associated steatotic liver disease (MASLD) predicts poor prognosis, the role of visceral adiposity and weight-based phenotypes is unclear. This study aimed to examine the clinical correlates of four unique MASLD phenotypes based on BMI and waist-to-height ratio (WHtR) and to assess the prognostic value of this classification with respect to mortality.Design Population-based cross-sectional study with prospective mortality follow-up.Setting Nationally representative US population derived from the National Health and Nutrition Examination Survey (NHANES), 2007–2018.Participants 6300 adults with MASLD, sampled from NHANES 2007 to 2018.Methods Participants were allocated into four phenotypes based on obesity/lean and central adiposity status. Obesity was defined as BMI≥30 kg/m², while those without obesity were determined as lean. High central adiposity was defined as WHtR≥0.6.Primary outcome measure The primary outcome was all-cause mortality. This was ascertained through National Death Index linkage with follow-up censored at 31 December 2019.Results Among adults with MASLD, 72.1% had obesity with high central adiposity (mean follow-up: 6.9±3.5 years). The non-obesity, high central adiposity phenotype (9.6%) exhibited the highest rates of hypertension (60.2%), type 2 diabetes (31.8%) and chronic kidney disease (23.3%). This phenotype portended the poorest 6-year survival (p<0.001) and was independently associated with higher all-cause mortality (adjusted HR (aHR) 3.1, 95% CI 1.3 to 7.7). A J-shaped association was observed between BMI and mortality, with increasing WHtR linked to elevated risk.Conclusion Non-obesity MASLD with high central adiposity confers the poorest survival and cardiometabolic burden.