Immuno-oncology biomarkers are promising tools for cancer risk assessment and early detection. To identify and validate their associations with breast cancer, nested case-control studies within the Taizhou Longitudinal Study (TZL) cohort and the UK biobank Pharma Proteomics Project (UKB-PPP) were conducted, comprising 195 and 881 incident breast cancer patients, together with their matched controls. Among the 92 plasma proteins tested by the Olink Immuno-oncology panel, 11 proteins were associated with breast cancer risk in the TZL cohort after multiple testing correction. Notably, hepatocyte growth factor (HGF) was validated in the UKB-PPP, particularly among postmenopausal women (OR = 1.13, 95% CI = 1.03-1.24). The association was stronger with Estrogen Receptor-negative breast cancer and confirmed by Mendelian Randomization analysis. Additionally, HGF mediated the effects of Healthy Lifestyle Index (27.17%) and BMI (19.79%) on breast cancer risk. Therefore, HGF could be an intervention target by either medicines or lifestyle changes to improve the prevention and treatment of breast cancer.