The diagnosis of cancer can motivate survivors to alter their lifestyle habits. Healthcare providers need to be aware of what changes patients are likely to make in order to derive more pertinent recommendations; however, few studies have reported pre- and post-diagnostic lifestyle behaviours. Semi-quantitative food frequency questionnaires (FFQs) completed approximately 1 year after diagnosis were used to evaluate dietary intake and supplement use before and after diagnosis in a cohort of 1,560 breast cancer patients participating in the UK, prospective DietCompLyf study. Intake of fruit and vegetables, wholegrains and lean sources of protein increased significantly post-diagnosis (P < 0.05, each). Conversely, after diagnosis consumption of high-fat, high-sugar products, red meat, coffee, some alcoholic drinks and refined grains significantly decreased (P < 0.05, each). Post-diagnostic changes in diet were accompanied by changes in the intake of macronutrients and a number of vitamins and minerals. Supplement use was highly prevalent (56.1%) pre-diagnosis, increasing to 62.8% after diagnosis (P = 0.001). Fish oils, multivitamin and minerals, and evening primrose oil were most often used and the proportion of users significantly increased (P < 0.05, each) after diagnosis. The percentage of women using oestrogenic botanical supplements (OBSs) was small but more than doubled to 8.4% after diagnosis (P < 0.05). British women participating in the DietCompLyf study reported significant changes in dietary intake and supplement use after their breast cancer diagnosis. These findings contribute to our understanding of female cancer survivors' dietary behaviours which is crucial for developing and implementing recommendations.