This paper analyzes John Stuart Mill’s stand towards the American Civil War and slavery, taking as a background his views on human nature and on the role played by public opinion, as well as his political concerns about the future of democracy. At first, when the majority of the English supported the Confederates, Mill’s efforts were directed at altering this stand, which he believed dishonored the name of England. Afterwards, his attention turned to the conditions necessary to guarantee the effective emancipation of Blacks in America after a Union victory. The result of this engagement was the production of extensive war-related material—essays, newspaper articles, and numerous private letters he exchanged with English and American correspondents. Although this material is fragmentary and heterogeneous, when interpreted in the broader context of his social views it reveals not only the importance that the issue of slavery had for him but also the complex views he had of this phenomenon and how to eliminate it—views that involved philosophical, economic, sociological, political, and, above all, moral aspects.