Important changes in symptom profiles occur early in the course of severe mental illness (SMI) after its clinical presentation. The emergence of negative symptoms is of particular concern for later function, and these may well have strong links with loneliness and poor social support which are also recognised to be common. However, the impact of loneliness and social isolation on symptoms and treatment outcomes remains unclear. To explore this, we conducted a retrospective observational analysis using data from electronic health records of a large mental health provider in London. Loneliness, living alone and symptoms of SMI were extracted using bespoke natural language processing algorithms. Symptoms were grouped into five domains: positive, negative, disorganised, manic, and catatonic, and were measured at diagnosis and 12 months following the initial SMI diagnosis. Loneliness and living alone were operationalised as binary variables, based on any recording during the follow-up period. A total of 8237 records were analysed. The prevalence of loneliness was 20.4 % and living alone 19.9 %, with only 6.8 % recorded with both. Recorded loneliness was associated with an increase in negative symptoms over the follow-up period, in models adjusted by sociodemographic variables and pharmacological treatment [OR 1.28 (1.11-1.48), p = 0.001], as was living alone to a weaker extent [OR 1.18 (1.02-1.36), p = 0.030]. No associations were found between either exposure variable and changes in symptoms from other domains. Given that negative symptoms are disabling and challenging to treat, these results highlight the importance of effective interventions to enhance social support in SMI over the early post-diagnostic period.