Mason wrote the Fugitive Slave Law of 1850, arguably the most hated and openly-evaded Federal legislation in U.S. history. The whole idea of using "popular sovereignty" as a means to expand slavery into the Western territories, starting with Kansas, leading to the Kansas-Nebraska Act and the violence of the Bleeding Kansas period, was hatched in Mason's Washington boarding house. Mason also was the chair of the ad-hoc Senate committee that investigated John Brown's raid on Harpers Ferry, and wrote its report, informally known as the Mason Report. He believed that Brown had been put up to his raid by Northern abolitionists, and was disappointed when his committee, after six months of work, could not find any basis to dispute Brown's statement that it was his own idea.