From its conceptual origin in Magna Charta, due process of law has required that government can deprive persons of rights only pursuant to a coordinated effort of separate institutions that make, execute, and adjudicate claims under the law. Originalist debates about whether the Fifth or Fourteenth Amendments were understood to entail modern “substantive due process” have obscured the way that many American lawyers and courts understood due process to limit the legislature from the Revolutionary era through the Civil War. They understood due process to prohibit legislatures from directly depriving persons of rights, especially vested property rights, because it was a court’s role to do so pursuant to established and general law. This principle was applied against insufficiently general and prospective legislative acts under a variety of state and federal constitutional provisions through the antebellum era. Contrary to the claims of some scholars, however, there was virtually no precedent before the Fourteenth Amendment for invalidating laws that restricted liberty or the use of property. Contemporary resorts to originalism to support modern substantive due process doctrines are therefore misplaced. Understanding due process as a particular instantiation of separation of powers does, however, shed new light on a number of key twentieth-century cases which have not been fully analyzed under the requirements of due process of law.