After arriving in the U.S., Realf explored the slums of New York City, became a Five Points missionary, and assisted in establishing there a course of cheap lectures and a self-improvement association. In 1856 he accompanied a party of free-state emigrants to Kansas, where he became a journalist and correspondent of several Eastern newspapers. He made the acquaintance of John Brown, accompanied him to Chatham, Ontario, Canada, and was to be Secretary of State in the provisional government that Brown projected. The movement being deferred for two years, Realf made a visit to England and a tour in the Southern states. When Brown made his attack on Harpers Ferry in October 1859, Realf was in Texas, where he was arrested and sent to Washington, D.C., being in imminent danger of lynching on the way.