BACKGROUND: Obesity is recognized as a risk factor for several cancer types and is linked with different patient outcomes. However, the extent to which obesity prevalence at treatment initiation differs from lifetime (ever) obesity exposure remains unclear. MATERIALS AND METHODS: In this descriptive real-world study, we analysed longitudinal body mass index (BMI) records from the QResearch general practice database linked to the National Health Service England Systemic Anti-Cancer Therapy dataset including patients with a first systemic treatment in 2013-2023. We calculated age-standardised obesity prevalence at first treatment and lifetime obesity prevalence based on BMI ≥30 kg/m2 at treatment initiation and from historic measurements for patients across 13 cancer types. RESULTS: In total, 79 271 patients were included (median age 66.5 years at first treatment, 54.2% female, and 89.0% white ethnicity). Age-standardised obesity prevalence at first treatment was 26.4% [95% confidence interval (CI) 26.0% to 26.9%] for all cancers, ranging from 13.7% (95% CI 11.6% to 15.9%) for pancreatic cancer to 36.3% (95% CI 29.9% to 42.7%) for uterine cancer. Lifetime obesity prevalence was 53.5% (95% CI 53.2% to 53.9%) for all cancers, ranging from 51.1% (95% CI 50.3% to 52.0%) for lung cancer to 63.0% (95% CI 58.8% to 67.2%) for hepatocellular carcinoma. CONCLUSIONS: We found that approximately one in four cancer patients in England were living with obesity at the start of systemic treatment, while half had a history of obesity. Reliance on BMI at treatment initiation for cancer prognostication substantially underestimates lifetime exposure to obesity with implications for precision medicine and outcomes research.