Existing drug therapies for hepatocellular carcinoma (HCC), including sorafenib, extend patient survival by only three months. We sought to identify novel druggable targets for use in combination with sorafenib to increase its efficacy. We implemented an in vivo genetic screening paradigm utilizing a library of 43 genes-of-interest expressed in the context of repopulation of the injured livers of Fumarylacetoacetate Hydrolase-deficient (Fah−/−) mice, which led to highly penetrant HCC. We then treated mice with vehicle or sorafenib to discover genetic determinants of sensitivity and resistance. Liver X Receptor alpha (LXRα) emerged as a potential target. To examine LXRα agonism in combination with sorafenib treatment, we added varying concentrations of sorafenib and LXRα agonist drugs to HCC cell lines. We performed transcriptomic analysis to elucidate the mechanisms of HCC death. Fah−/− mice injected with the screening library developed HCC tumor clones containing Myc cDNA plus various other cDNAs. Treatment with sorafenib resulted in sorafenib-resistant HCCs that were significantly depleted in Nr1h3 cDNA, encoding LXRα, suggesting that LXRα activation is incompatible with tumor growth in the presence of sorafenib treatment in vivo. The combination of sorafenib and LXR agonism led to enhanced cell death as compared to monotherapy in multiple HCC cell lines, due to reduced expression of cell cycle regulators and increased expression of genes associated with apoptosis. Combination therapy also enhanced cell death in a sorafenib-resistant primary human HCC cell line. Our novel in vivo screen led to the discovery that LXR agonist drugs potentiate the efficacy of sorafenib in treating HCC.