During the twentieth century a flurry of historical and theoretical studies contributed to a renewed and sustained scholarship on English republicanism and the political thought of Algernon Sidney (1623-1683). This literature on Sidney has predominantly been in English, French, Italian, and Portuguese, with further contributions in Dutch and German. Despite the fruits of this voluminous attention, however, the various assumptions of these opposing historiographies – not always in conversation with one another – have often served to obscure, rather than illuminate, what Sidney himself was saying and doing in his work. This thesis seeks to reassess the thought of Sidney in reference to his major works, Court Maxims [1664-1665](1996) and Discourses Concerning Government [1681-1683](1698), but also his overlooked essay Of Love [1650s](1748) and a range of neglected and fresh sources, especially his correspondence. Indeed, utilising an array of sources beyond his works, I also re-examine Sidney’s life and activities, particularly his time as ambassador from the English republic to the monarchies of Denmark-Norway and Sweden (1659-1660). Introducing a significant collection of new manuscript material by Sidney, my first chapter investigates Sidney’s earlier political thinking as a republican magistrate, often intentionally adapted and utilised as an active tool of diplomacy, and the ways in which this early thought is developed into his later writings on government, which is what I reconsider across the remaining six chapters. Crucial to my reassessment of Sidney’s political thought is a conceptual reconstruction, deeper than hitherto attempted, of his vision of fallen and bipartite human nature – body and soul, passion and reason, will and understanding, sense and cognition, vice and virtue – within the time and history of the saeculum. States, like humans, are mortal and exist in historical time, an insight that runs throughout Sidney’s various accounts on the life and death of states, from Greece and Rome to contemporary England. I argue that by closely exploring Sidney’s comprehension of human nature it becomes possible to envision how he proposes not only a synthesis of republicanism and natural law, including virtue ethics and natural rights, but also his Reformed beliefs and what can be called a rationalist political thinking that centres temporal liberty as a right and the end of civil government. Liberty conceived in neo-Roman terms, not virtue, is the essential ordering principle of Sidney’s politics, with virtue redescribed as a supporting force. Accordingly, Sidney cannot be adequately described as Aristotelian or Platonist without serious qualification – something he makes clear in his own words, as I demonstrate, because such perfectibility is beyond mortal beings fallen in time. Moreover, it also becomes apparent that Sidney’s forays into English constitutional historiography, far from contradicting his other arguments, also cohere into his understanding of natural law and temporality. Sidney might appear an eclectic thinker, but he is not as inconsistent as he is often portrayed. Ironically, it is precisely the gravity of Sidney’s idiosyncratic theological beliefs that lead him to not only forsake the extremes of Greek virtue politics, but also adopt a rationalist political thinking later mistaken as irreligious. In a sense, therefore, this thesis also offers to demonstrate the genesis of a confusion and series of false binaries that came to define the contours of the interconnected and opposing historiographies of Sidney from his execution to the present day.