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), who was a well-known scholar. As a soldier in the troops of the Electorate of Saxony Justi participated in the campaigns to Bohemia and Moravia. Lieutenant Colonel Wigand Gottlob von Gersdorff (? – 1745) recognized Justi's talent, encouraged him to study and made him to his own private secretary. Justi enrolled at the University of Wittenberg to study law under the supervision of the Thomasian 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 this very 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 upon to be chair of German eloquence at the Theresian Academy (Theresianum) in Vienna, Maria Theresa’s new ‘imperial academy’ for recruits to the civil service. Contemporaries argued that Justi had converted for this position. The fact that Justi's daughter Carolina was baptized in St Stephen's cathedral in Vienna may be interpreted as an indication that Justi had converted. After his departure from Vienna (1753) Justi repeatedly denied that he had ever converted, and in his publications he took often 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 by 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 on 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 transformation from natural law to cameral sciences. For Justi, the field of application of natural law was comparatively narrow in relation to social and economic policy (Policey) encompassing all the activities of the state. Justi elaborated on Policey in his works on the science of police (Policey-Wissenschaft). For Justi it was not the 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 coming 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 Natur und Wesen der Staaten published with Scheidemantel's annotations, while the latter reviewed the same work favourably in his Berichtugungen. 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’s natural law has been interpreted in three different ways. First, Justi has been considered as a follower of Christian Wolff’s conception of law of nature and of law of nations whereby perfection and happiness are the central concepts (Jürgen Backhaus). Second, Justi’s natural law has been interpreted as a failed attempt to reconcile the main concepts of Christian Thomasius’s and Christian Wolff’s natural law. Hence, Justi would have tried 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 theorization of cameral sciences was linked with replacing the Aristotelian tradition and Wolffianism with new empirically motivated natural law that gave little room for metaphysical speculations. Closer analysis reveals that von Justi was one of the most ardent critics of Wolff’s philosophy, including Wolff’s views on natural law. Von Justi namely supported “natural law of instincts”. He had studied Johann Jacob Schmauss’s Neues systema in detail (Schmauss 1754). In his Natur und Wesen der Staaten (1760), he stated that despite the devastating critiques levelled against it, the main concept of Schmauss’s natural jurisprudence was correct (Justi (1760) 1771: 384–385). In the footsteps of 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 beings. Correspondingly he legitimized the pursuit for self-interest (das eigene Interesse) since it was a significant moving force for the commerce, consumption and different type of occupations. For this very reason, Justi argued, self-interest should not be regulated too much. Citizens as reasonable beings have the right to guide themselves to their own happiness.