From that point, Smith was a committed abolitionist. Smith served as president of the New York Anti-Slavery Society from 1836 to 1839 and gradually became more and more outspoken in his denunciation of slavery. He began publicly encouraging opponents of slavery to assist runaway slaves, an act which was illegal at the time. He also donated considerable money to the antislavery cause.
In some cases, Smith purchased slaves and their families for the purpose of freeing them. He also donated money to abolitionists for their travel expenses and publications. It's estimated that over the course of his lifetime, Smith gave away more than eight million dollars, an astonishing sum in the mid-1800s.
Smith's home also became a widely recognized stop on the Underground Railroad. Numerous runaway slaves benefited from Smith's hospitality, food, and money, on their way to Canada. The Smith family owned a substantial amount of land in New York, and Smith sold numerous tracts to former slaves, some of whom he had aided in their escape, for one dollar. Roughly 140,000 acres were transferred to African Americans in this way from 1846 to 1850.