While scholars have focused on the importance of the landmark decisions of the United States Supreme Court and its Chief Justice, John Marshall, in the rising influence of the federal justice system in the early Republic, the crucial role of the circuit courts in establishing uniformity of federal law and procedure across the nation has largely been ignored. This thesis seeks to remedy this lack of research on circuit courts by revealing the central role of their presiding Supreme Court justices in the successful development of a national court system drawn up from the ‘inferior’ courts rather than down from the Supreme Court to the lower jurisdictions. This thesis argues that, at a time when the Supreme Court had few cases to consider, all of the nation’s law was formulated by the lower courts; with very few decisions appealed, the circuit court opinions were invariably accepted as final, settling the law for each circuit and for the nation if followed by other justices. Therefore, in the early years, it was the circuit experience and not Supreme Court authority which shaped the course of United States law. This thesis contributes to an understanding of this early justice system because of its focus on and the depth of its research into the work of the circuit courts. Through detailed analysis, it reveals the sources used by the justices to influence the direction of the law and, by its reading of almost 2000 cases tried by four prominent Marshall associate justices, presents insights into momentous issues facing the Union. The thesis examines the generality of the circuit work of each justice but pays particular attention to the different ways in which each contributed to the shaping of United States law. Understanding the importance of the role of the circuit courts leads to a more informed reading of early American legal history.