Severe mental illness is characterised by a 20-year mortality gap due to cardiometabolic disease. Poor diet in those with severe mental illness is an important and modifiable risk factor. The present study aimed to (i) examine baseline nutritional intake in youth with first-episode psychosis (FEP), (ii) evaluate the feasibility and acceptability of nutritional intervention early in FEP and (iii) to evaluate the effectiveness of early dietary intervention on key nutritional end points. Participants were recruited over a 12-month period from a community-based programme specifically targeting young people aged 15–25 years with newly diagnosed FEP. Individual dietetic consultations and practical group sessions were offered as part of a broader lifestyle programme. Dietary assessments were conducted before and at the end of the 12-week intervention. Participants exceeded recommended energy and Na intakes at baseline. Retention within the nutrition intervention was 67 %, consistent with other interventions offered to FEP clients. There was a 47 % reduction in discretionary food intake (−94 g/d, P<0·001) and reductions in daily energy (−24 %, P<0·001) and Na (−26 %, P<0·001) intakes. Diet quality significantly improved, and the mean change was 3·6 (95 % CI 0·2, 6·9, P<0·05), although this finding was not significant after Bonferroni’s correction. Increased vegetable intake was the main factor contributing to improved diet quality. Nutrition intervention delivered shortly after initiation of antipsychotic medication is feasible, acceptable and effective in youth with FEP. Strategies to prevent weight gain and metabolic decline will contribute to prevent premature cardiometabolic disease in this vulnerable population.