People with severe mental illness (SMI) have numerous risk factors that may predispose them to food insecurity (FI); however, the prevalence of FI and its effects on health are under-researched in this population. The present study aimed to describe the prevalence of FI and its relationship to lifestyle factors in people with SMI. This cross-sectional study recruited people with SMI receiving long-acting injectable (LAI) antipsychotic medication from community services at three sites in Sydney, Australia. Assessments were completed on physical health and lifestyle factors. χ2 Tests, independent-samples t tests and binary logistic regression analyses were calculated to examine relationships between lifestyle factors and FI. In total, 233 people completed the assessments: 154 were males (66 %), mean age 44·8 (sd 12·7) years, and the majority (70 %) had a diagnosis of schizophrenia. FI was present in 104 participants (45 %). People with FI were less likely to consume fruits (OR 0·42, 95 % CI 0·24, 0·74, P = 0·003), vegetables (OR 0·39, 95 % CI 0·22, 0·69, P = 0·001) and protein-based foods (OR 0·45, 95 % CI 0·25, 0·83, P = 0·011) at least once daily, engaged in less moderate to vigorous physical activity (min) (OR 0·997, 95 % CI 0·993, 1·000, P = 0·044), and were more likely to smoke (OR 1·89, 95 % CI 1·08, 3·32, P = 0·026). FI is highly prevalent among people with SMI receiving LAI antipsychotic medications. Food-insecure people with SMI engage in less healthy lifestyle behaviours, increasing the risk of future non-communicable disease.