Background: Insulin-like growth factor-1 (IGF1) has been implicated in mitogenic and anti-apoptotic mechanisms that promote susceptibility to cancer development and growth. Previous epidemiological studies have described phenotypic associations between higher circulating levels of IGF1 in adults with higher risks for breast, prostate, ovarian, colorectal, melanoma and lung cancers. However, such evidence is prone to confounding and reverse causality. Furthermore, it is unclear whether IGF1 promotes only the survival and proliferation of cancerous cells, or also the malignant transformation of healthy cells. Methods: We perform a genome-wide association study in 428,525 white European ancestry individuals in the UK Biobank study (UKBB) and identify 831 independent genetic determinants of circulating IGF1 levels, double the number previously reported. Results: Collectively these signals explain ~7.5% of the variance in circulating IGF1 levels in EPIC-Norfolk, with individuals in the highest 10% of genetic risk exhibiting ~1 SD higher levels than those in the lowest 10%. Using a Mendelian randomization approach, we demonstrate that genetically higher circulating IGF1 levels are associated with greater likelihood of mosaic loss of chromosome Y in leukocytes in men in UKBB (OR per +1 SD = 1.038 (95% CI: 1.010-1.067), P=0.008) and 23andMe, Inc. (P=6.8×10-05), a biomarker of genomic instability involved in early tumorigenesis. Genetically higher IGF1 is also associated with higher risks for colorectal (OR = 1.126 (1.048-1.210), P=1.3×10-03) and breast cancer (OR= 1.075 (1.048-1.103), P=3.9×10-08), with similar effects on estrogen positive (ER+) (OR = 1.069 (1.037-1.102), P=2.3×10-05) and estrogen negative (ER-) (OR = 1.074 (1.025-1.125), P=3.9×10-08) subtypes. Conclusions: These findings give an insight into the genetic regulation of circulating IGF1 levels and support a causal role for IGF1 in early tumorigenesis and risks for breast and colorectal cancers.