Justi was born in Brücken an der Helme (Thuringia). He studied at the Quedlinburg Gymnasium, which at that time was directed by Tobias Eckhard (1662–1737), a well-known scholar. As a soldier in the troops of the Electorate of Saxony Justi participated in campaigns to Bohemia and Moravia. Lieutenant Colonel Wigand Gottlob von Gersdorff (? – 1745) recognized Justi's talent, encouraged him to study and made him his own private secretary. Justi enrolled at the University of Wittenberg to study law under the supervision of  Augustin Leyser (1683–1752). Justi defended a thesis on the punishment of deserters, De Fuga Militae,in 1744. His first literary success was a satire called Dichterinsel (1745). The same year he began publishing his first journal and from that time he was always managing at least one journal. In 1747 Justi won an essay prize contest set by the Royal Prussian Academy of Sciences with his text criticizing Leibniz’s and Christian Wolff’s theory of monads. In the same year Justi had been appointed to the service of the widowed Duchess of Sachsen-Eisenach Anna Sophie Charlotte von Sachsen-Eisenach (1706–1751). Three years later, in 1750, Justi was called to the chair of German eloquence at the Theresian Academy (Theresianum) in Vienna, Maria Theresa’s new ‘imperial academy’ for recruits to the civil service. Contemporaries suggested that Justi had converted for the sake of this position. The fact that Justi's daughter Carolina was baptized in St Stephen's cathedral in Vienna may be confirmation of his conversion, but after his departure from Vienna (1753) Justi repeatedly denied converting, and his publications often took an anti-Catholic stand. From 1752 he became the academy's professor of Praxis im Cameral- Commercial- und Bergwesen. In the following year Justi moved to Leipzig and from there in 1755 to Göttingen, where he became Ober-Policey-Commissar and the first lecturer to teach cameral sciences at the University of Göttingen. Soon after, in 1757, Justi left Göttingen and went to work for the Danish Court. After a short period in Denmark, Justi supported himself until 1765 as an independent writer. This was the time when he wrote most actively. He was granted a pension from Prussia for his pro-Prussian and pro-English pamphlets. In 1765 he was appointed to an inspectorate of mines, glass- and steelworks (Berghauptmann) in Prussia. Three years later he was accused of having misused the state’s money. Justi died in 1771, nearly blind and accused of embezzlement, imprisoned in the vicinity of fortress of Küstrin.

There exists no picture of Justi. Justi argued that he did not want to waste his time being portrayed. It is worth noting that Justi neither came from a noble family nor was he ever ennobled. From the early 1750s onward he started to call himself von Justi instead of Justi.

Justi’s main works related to natural law are Grundriß einer guten Regierung (1759) and Natur und Wesen der Staaten (1760). Both works were written after Justi’s stay in Göttingen (1755-1757) when he was looking for a position in Prussian service. In these works, Justi elaborated his views on natural law in the context of discussing the origin of all sciences of the state. According to Justi, natural law was to provide the philosophical foundation of cameral sciences. At the same time these works represent a departure from natural law towards cameral sciences, where the focus was increasingly on economic arguments. For Justi, natural law was a fairly narrow discipline compared with the science of police (Policey-Wissenschaft), which encompassed all state activities including social and economic policy. For Justi it was not lawyers, but cameralists, who would be needed in the future. Therefore, he even suggested founding a faculty of cameral sciences. However, the philosophical justification of this faculty would still be strengthened with arguments from natural law.

Justi’s Natur und Wesen der Staaten found its most favourable readers in Heinrich Gottfried Scheidemantel (1739–1788) and Johann Friedrich von Pfeiffer (1718–1787). The former was responsible for the second edition of Justi’s workpublished with Scheidemantel's annotations, while the latter reviewed the same work favourably in his Berichtigungen berühmter Staats-, Finanz-, Policei-, Cameral-, Commerz- und ökonomischer Schriften dieses Jahrhunderts (vol. 2, 1782). Not unlike Justi, von Pfeiffer was an advocate of universal cameral sciences encompassing all the sciences of the state with a foundation in natural law. There are Russian and Dutch translations of Justi’s Natur und Wesen der Staaten and it seems to have played role in informing the conceptual apparatus of social and economic reforms under Catherine the Great (Nakaz).

Justi never taught natural law anywhere, nor has he written a natural law textbook. In this sense he was more of a marginal natural law writer. Nevertheless, his natural law has been a topic of considerable scholarly interest. There are two reasons for this. First, the basic assumptions of Justi and his contemporary cameralists were based on natural law. Hence, the interpretations of Justi’s natural law are important for the general interpretations of cameral sciences. Second, Justi’s Natur und Wesen der Staaten focused primarily on one aspect of natural law, namely “General State Law” (Allgemeines Staatsrecht). Therefore, it is a key text for interpreting Justi’s political thought. Politically, Justi has been regarded as an absolutist (Jutta Brückner), as a liberal (Marcus Obert, Uwe Wilhelm), and as an advocate of a limited modern commercial monarchy (Ulrich Adam, Ere Nokkala).   

Justi’s natural law has been interpreted in three different ways. First, as following Christian Wolff’s conception whereby perfection and happiness are the central concepts (Jürgen Backhaus). Second, as a failed attempt to reconcile the main concepts of Christian Thomasius’s and Christian Wolff’s natural law, trying to combine self-preservation and happiness under one umbrella (Jutta Brückner). Third, Justi has been interpreted as an original thinker who was an advocate of the natural law of instincts (Horst Dreitzel, Merio Scattola, Ere Nokkala).

The theory of cameral sciences was meant to replace the Aristotelian tradition as well as Wolffianism with a new empirically based natural law that left little room for metaphysical speculations. Justi was in fact one of the most ardent critics of Wolff’s philosophy, including Wolff’s views on natural law, and on paternal government. He adopted the “natural law of instincts” that he had studied in detail in Johann Jacob Schmauss’s Neues systema (1754). In his Natur und Wesen der Staaten (1760), he maintained that despite the severe criticism levelled against it, the main concept of Schmauss’s natural jurisprudence was correct (Justi (1760) 1771: 384–385). Like his Göttingen colleague, Justi argued that the moral actions of man were not in the first place motivated by reason, but by passions, instincts and interests. Those were the mainsprings of human activity. Correspondingly he legitimized the pursuit of self-interest (das eigene Interesse) since it was the moving force behind commerce, consumption and the variety of human occupations. Accordingly, self-interest should not be over-regulated. As reasonable beings citizens have the right to guide themselves to their own happiness.