On Monday 22 June 1772, the English jurist William Murray, 1st Earl of Mansfield, delivered his oral verdict as Chief Justice of the Court of King’s Bench in the famous case involving the enslaved Afro-British servant James Somerset to declare that only an Act of Parliament could legalize domestic bondage and that Somerset was a free man. For the estimated 15,000 captives living in the English metropole, Somerset v. Stewart effectively undercut the Anglo-Atlantic slavocracy that had hid behind legal technicalities and extrajudicial decrees defending domestic bondage since the last quarter of the seventeenth century. In order to offer a full treatment of Somerset, its Afro-British legal antecedents, and the Black experience in Early Modern Britain, this work traces the roots of British racial construction--deep seated physiognomic, socio-cultural, legal, and economic roots that date to 1553 when the English first explored equatorial West Africa or what cartographers generically branded “Negroland.” When investigating Somerset scholars have overlooked the semantics of race, its longue durée link to English legal systems, and the historical actors who socially and legally defiled the Black presence in the British Empire. In addition to reconnoitering the origins of British racial construction, this work examines the judicial minutia of Afro-British case law and Mansfield’s 1772 decision, while offering a comprehensive account of its immediate and long-term effects on emancipations in the Anglo-American diaspora. This provides an all-inclusive treatment neglected by Somerset scholars. Mansfield’s verdict was an exceptional threat to slavery in that it resonated powerfully within interracial trans-Atlantic abolitionist movements and the enslaved communities that waged various forms of “diasporic warfare” against captivity throughout the British Empire. My original quantitative data based on the Glasgow University “Runaway Slave in Eighteenth-Century Britain project” reveals the correlation between pro- and anti-slavery Afro-British legal cases and the 830 ‘runaway’ and eighty-two ‘for sale’ advertisements published in eighteenth-century British newspapers. The quantitative evidence illustrates that from 1758 the surge of Afro-British ‘runaways’ led to the high-profile trials of Joseph Harvey (1762), Jonathan Strong (1765), and Thomas John Hylas (1768) which provoked increased anti-slavery activity the following decade. Indeed, by the 1760s servants were absconding in record numbers and resisting--as what I coin metropolitan maroons--and domestic slavery was quickly dying out in Britain. The public reaction to Mansfield’s 1772 verdict, coupled with the precipitous fall of post-Somerset ‘runaway’ and ‘for sale’ advertisements, proved the end of de facto slavery in England. While its legal legacies were at times ambiguous, the Somerset case gained new meanings in the imaginations of emancipationists and pro-slavery apologists alike, as tellings and retellings of its verdict were passed by word of mouth among enslaved people and through popular publications among literate free people in the decades that followed. Some of the reverberations were resounding and others much more subtle, yet all attest to the special significance of Somerset in the long emancipationist struggle against slavery.