Type 2 diabetes (T2D) and its hallmarks insulin resistance, impaired insulin secretion, and hyperglycaemia affect over 400 million persons worldwide and are associated with raised cardiovascular risk, but their causal role has been difficult to dissect due to overlap between risk factors. We used Mendelian randomization analysis, which utilises genetic polymorphisms associated with a risk factor, to assess causal effects of T2D, insulin resistance, insulin secretion, and fasting glucose on mortality, ischaemic stroke, and coronary artery disease (CAD) risk in 120,091 adults in the UK Biobank and in the CARDIoGRAMplusC4D consortium (63,746 cases of CAD and 130,681 controls). We found evidence for a causal effect of T2D on raised CAD risk (odds ratio (OR) per doubling in the odds of T2D, 1.07, 95% confidence interval (CI) 1.05 – 1.09, P = 1.2 x 10-9) and for a causal effect of impaired insulin secretion on the risk of CAD (OR per SD-unit decrease, 1.14, 95% CI 1.06 – 1.22, P = 0.002). The genetic score for insulin resistance was associated with increased coronary artery disease risk; however, sensitivity analysis indicated that the instrument might not be appropriate to use for robust causal inference testing. Our results support previous reports of a causal role of T2D and impaired insulin secretion in coronary artery disease and point to a complex relationship between variants affecting insulin resistance and cardiovascular outcomes.