Background: Tobacco exposure causes 8 of 10 lung cancers, and identifying additional risk factors is challenging due to confounding introduced by smoking in traditional observational studies. Materials and Methods: We used Mendelian randomization (MR) to screen 207 metabolites for their role in lung cancer predisposition using independent genome-wide-association studies (GWAS) of blood metabolite levels (n =7,824) and lung cancer risk (n=29,266 cases / 56,450 controls). A nested case control study (656 cases and 1,309 matched controls) was subsequently performed using pre-diagnostic blood samples to validate MR association with lung cancer incidence data from population-based cohorts (EPIC and NSHDS). Results: An MR-based scan of 207 circulating metabolites for lung cancer risk identified that blood isovalerylcarnitine (IVC) was associated with a decreased odds of lung cancer after accounting for multiple testing (Log10-OR = 0.43, 95% CI: 0.29-0.63). Molar measurement of IVC in pre-diagnostic blood found similar results (Log10-OR = 0.39, 95% CI: 0.21-0.72). Results were consistent across lung cancer sub-types. Conclusions: Independent lines of evidence support an inverse association of elevated circulating IVC with lung cancer risk through a novel methodological approach that integrates genetic and traditional epidemiology to efficiently identify novel cancer biomarkers. Impact: Our results find compelling evidence in favor of a protective role for a circulating metabolite, IVC, in lung cancer aetiology. From the treatment of a Mendelian disease, isovaleric acidemia, we know that circulating IVC is modifiable through a restricted protein diet or glycine and L-carnatine supplementation. IVC may represent a modifiable and inversely associated biomarker for lung cancer.