Antebellum abolitionists participated—through correspondence, print, and travel—in extensive transatlantic reform networks and often considered themselves citizens of the world. William Lloyd Garrison, the radical antislavery editor, was at the center of such networks and printed the same cosmopolitan slogan on every issue of his Boston newspaper, the Liberator: “Our Country is the World—Our Countrymen are All Mankind.” By focusing on the public and private writings of the Garrisonians—the antislavery radicals who took their name from Garrison—this dissertation shows how transnational reform networks functioned as communities of discourse in which the abolitionists developed radical ideas about slavery, democratic politics, nations, and patriotism. The Garrisonians’ transatlantic friendships, many of which were forged at a “World’s Convention” on slavery held in London in 1840, brought abolitionists into contact with numerous European radicals, including Chartists, free traders, Irish Repealers, and revolutionaries like the Italian Giuseppe Mazzini and the Hungarian Lajos Kossuth. Interpreting their networks in light of a broadly Romantic worldview, the Garrisonians were convinced that they were uniquely cosmopolitan figures. But the Garrisonians’ affinity with certain British reformers also reveals that they were more similar to other antebellum reformers than previously thought. Though often seen as the anti-political pariahs of the antislavery movement, the Garrisonians’ endorsements of movements like Chartism and Irish Repeal suggest that they were more sensitive to political strategy than scholars have allowed, and that they belong within a transatlantic context of democratic politics. The Garrisonians’ transatlantic networks were also crucial to their development of forward-looking ideas about nations and patriotism. Garrisonians were “civic nationalists” who viewed nations as political, rather than racial or ethnic, communities, and they also articulated a version of “cosmopolitan patriotism,” which identified love for country with a willingness to criticize the vestiges of despotism in American institutions. But in contrast to exceptionalist narratives, which view the concept of “civic nationalism” as an inevitable outgrowth of the nation’s founding creeds, I argue that the Garrisonians’ ideas about nations were forged within transnational discursive communities, and were informed in part by encounters with European reformers.