Liebig was the son of a pigment and chemical manufacturer whose shop contained a small laboratory. As a youth, Liebig borrowed chemistry books from the royal library in Darmstadt and followed their “recipes” in experiments he conducted in his father's laboratory. At the age of 16, after studying pharmacy for six months under the tutelage of an apothecary at Heppenheim, he persuaded his father that he wanted to pursue chemistry, not the apothecary trade. In 1820 he began his study of chemistry with Karl Kastner at the Prussian University of Bonn, following Kastner to the University of Erlangen in Bavaria, where Liebig ultimately received his doctorate in 1822. His diligence and brilliance was noticed by the Grand Duke of Hesse-Darmstadt and his ministers, who funded his further chemistry studies under Joseph-Louis Gay-Lussac (Gay-Lussac, Joseph-Louis) in Paris between 1822 and 1824. While in Paris, Liebig investigated the dangerous explosive silver fulminate, a salt of fulminic acid. Concurrently, the German chemist Friedrich Wöhler (Wöhler, Friedrich) was analyzing cyanic acid. Liebig and Wöhler jointly realized that cyanic acid and fulminic acid represented two different compounds that had the same composition—that is, the same number and kind of atoms—but different chemical properties. This unexpected conclusion, which was later codified under the concept of isomerism by the Swedish chemist Jöns Jacob Berzelius (Berzelius, Jöns Jacob), led to a lifelong friendship between Liebig and Wöhler and to a remarkable collaborative research partnership, frequently conducted via correspondence.

Liebig's scientific work with fulminates, together with his fortunate meeting with the influential German naturalist and diplomat Alexander von Humboldt (Humboldt, Alexander von), who was always keen to patronize younger talent, led to Liebig's appointment at the small University of Giessen in May 1824. As Liebig later observed in his fragmentary autobiography, “at a larger university, or in a larger place, my energies would have been divided and dissipated, and it would have been much more difficult, perhaps impossible, to reach the goal at which I aimed.”

Liebig succeeded in institutionalizing the independent teaching of chemistry, which hitherto in German universities had been taught as an adjunct to pharmacy for apothecaries and physicians. Furthermore, he expanded the realm of chemistry teaching by formalizing a standard of training based upon practical laboratory experience and by focusing attention upon the uncultivated field of organic chemistry. The key to his success proved to be an improvement in the method of organic analysis. Liebig burned an organic compound with copper oxide and identified the oxidation products (water vapour and carbon dioxide) by weighing them, directly after absorption, in a tube of calcium chloride and in a specially designed five-bulb apparatus containing caustic potash. This procedure, perfected in 1831, allowed the carbon content of organic compounds to be determined to a greater precision than previously known. Moreover, his technique was simple and quick, allowing chemists to run six or seven analyses per day as opposed to that number per week with older methods. The rapid progress of organic chemistry witnessed in the early 1830s suggests that Liebig's technical breakthrough, rather than the abandonment of the belief that organic compounds might be under the control of “vital forces (vitalism),” was the key factor in the emergence of biochemistry and clinical chemistry. The five-bulb potash apparatus he designed for carbon dioxide absorption rapidly became, and remains to this day, emblematic of organic chemistry.

Liebig's introduction of this new method of analysis led to a decade of intensive investigation of organic compounds, both by Liebig and by his students. Liebig himself published an average of 30 papers a year between 1830 and 1840. Several of these investigative reports became highly significant to further developments in the theory and practice of organic chemistry. Most noteworthy among these writings were his series of papers on the nitrogen content of bases, joint work with Wöhler on the benzoyl radical (1832) and on the degradation products of urea (1837), the discovery of chloral (trichloroethanal, 1832), the identification of the ethyl radical (1834), the preparation of acetaldehyde (ethanal, 1835), and the hydrogen theory of organic acids (1838). He also popularized, but did not invent, the Liebig condenser, still used in laboratory distillations.

Liebig's analytical prowess, his reputation as a teacher, and the Hessian government's subsidy of his laboratory created a large influx of students to Giessen in the 1830s. Indeed, so many students were drawn to Liebig that he had to expand his facilities and systematize his training procedures. A considerable number of his students, some 10 per semester, were foreigners. Maintaining a devoted following among foreign audiences helped firmly to establish Liebig's emphasis on laboratory-based teaching and research in foreign countries and in other German states. For example, the Royal College of Chemistry founded in London in 1845, the Lawrence Scientific School established at Harvard University in 1847, and Hermann Kolbe (Kolbe, Hermann)'s large laboratory at Leipzig in Saxony in 1868 were all modeled upon Liebig's program.

One of the major investigations that Liebig collaboratively pursued with Wöhler was an analysis of the oil of bitter almonds in 1832. After demonstrating that the oil could be oxidized to benzoic acid (benzenecarboxylic acid), the two chemists postulated that both substances, as well as a large number of derivatives, contained a common group, or “ radical,” which they named “benzoyl.” This research, based upon Swedish chemist Jöns Jacob Berzelius (Berzelius, Jöns Jacob)'s electrochemical and dualistic model of inorganic composition, proved to be a landmark in classifying organic compounds according to their constituent radicals.

The radical theory, together with a large accumulation of data from organic analysis experiments, provided Liebig and Wöhler sufficient background to begin to analyze the complex organic compounds in urine. Between 1837 and 1838 they identified, analyzed, and classified many of the constituents and degradation products of urine, including urea (carbamide), uric acid, allantoin, and uramil. Among their conclusions, uramil was reported to be produced by “innumerable metamorphoses” of uric acid—itself a degradation product, they conjectured, of flesh and blood. This magnificent investigation, which astonished British chemists when Liebig reported it to the British Association for the Advancement of Science during a visit to Britain in 1837, gave contemporary physicians new insight into the pathology of many kidney and urinary bladder diseases. Later, in 1852, Liebig provided physicians with simple chemical procedures whereby they could quantitatively determine the amount of urea in urine. In another work of practical use to physicians, he determined the oxygen content of the air by quantifying its adsorption in an alkaline solution of pyrogallol (benzene-1,2,3-triol).

Liebig's realization that organic chemistry could be used as a tool to investigate living processes led him to abandon pure chemistry in 1840. In that year he published Die organische Chemie in ihrer Anwendung auf Agricultur und Physiologie (Chemistry in Its Applications to Agriculture and Physiology). In this German publication, which soon appeared in English and French translations, Liebig claimed that because “perfect agriculture (agricultural technology) is the true foundation of all trade and industry,” a “rational system of agriculture cannot be formed without the application of scientific principles.” Only the chemist, he argued dogmatically, could tell the farmer the best means of feeding plants, the nature of the different soils, and the action of particular manures upon them. By analyzing soils, Liebig showed that the prevailing “humus theory” in which a plant's carbon content was claimed to have originated principally from leaf mould, and not from atmospheric photosynthesis, was fallacious. On the other hand, Liebig argued incorrectly for years that atmospheric ammonia and nitrates in the soil were more important direct sources of plant nitrogen than manures, whose principal function he viewed as providing trace minerals from the products of decomposition that remained in the soil. In order to provide these minerals more efficiently, Liebig began to develop “chemical manures” in 1845. Although Liebig's claim was later proven to be incorrect, and his fertilizers were shown to be inefficient and uneconomic, investigations conducted at the Rothamsted Experimental Station in Hertfordshire by his English pupil J.H. Gilbert (Gilbert, Sir Henry), together with the landowner John Bennet Lawes (Lawes, Sir John Bennet, 1st Baronet), led to the discovery of superphosphates, which were readily developed as fertilizers (fertilizer).

Sulfuric acid production for fertilizers accelerated both the industrialization of Europe and the vertical integration of chemical industries. Liebig's aphorism of 1843, that the measure of a country's civilization lay in the amount of sulfuric acid it consumes every year, became widely known. Both directly and indirectly, Liebig was an influential figure in the development of scientific agriculture and, thus, in increasing food production at a time when a rising European population was undergoing vast urban and industrial expansion.

In 1842 Liebig published a sequel, Die organische Chemie in ihrer Anwendung auf Physiologie und Pathologie (Animal Chemistry or Organic Chemistry in Its Applications to Physiology and Pathology), which is considered to be a foundational writing of modern biochemistry. In this work, Liebig employed analyses and highly speculative equations in an attempt to unravel the metabolic routes by which foodstuffs were transformed into flesh and blood and whereby tissues were degraded into animal heat, muscular work, and secretions and excretions. Although many of the details were later shown to be wrong, his novel approach of examining metabolism from a chemical viewpoint inspired decades of further research. A false hypothesis in science can often be fruitful; by demonstrating the errors of Liebig's schemes, many important principles were discovered. For instance, Liebig was wrong in claiming that fermentation and putrefaction were merely dynamic reshufflings of the constituent parts of chemical substances; yet his claim prompted many physicians to espouse a chemical theory of disease that challenged the predominant sanitarian view that disease was spread by the poisonous miasma that arose from accumulated sewage.

Liebig grew increasingly interested in the chemistry of food, especially in discovering better ways to cook meat in order to preserve its nutritional qualities. In his 1847 publication Chemische Untersuchung über das Fleisch (Research on the Chemistry of Food), Liebig described a particular “extract of meat” prepared by low-pressure evaporation of the soup from lean meat, and he claimed it to be a valuable restorative for the sick, wounded, and ill-nourished. In later editions of his popular Chemische Briefe (Familiar Letters on Chemistry), he pointed out that in countries such as South America and Australia, where cattle were routinely slaughtered for their hides or tallow, his meat extract could be prepared extremely economically. Belgian railway engineer Georg Giebert followed up this suggestion and, in 1865, began to market, with Liebig's promotional assistance, Liebig's extract of meat as a nutritious food for invalids and the labouring classes. In the same decade Liebig also improved the commercial processing of artificial milk for infants, the baking of whole-meal bread, and the silvering of mirrors.

Liebig remained in Giessen for 28 years, where the Duke of Hesse-Darmstadt made him a baron in 1845. In 1852, fatigued from teaching, he moved to the University of Munich, where he no longer offered practical instruction but pursued his own interests and concentrated upon popular lecturing and writing. Through the popularity of his Familiar Letters on Chemistry, he became viewed as an elder statesman of science, and he regularly commented on broader issues including scientific methodology, the opposition to materialism, and the dangers of failing to recycle sewage or replace soil nutrients that were harvested as animal and human food.

Liebig was frequently hot-tempered and quarrelsome by nature, and he tenaciously upheld his own particular viewpoints. As editor of the monthly Annalen der Pharmacie und Chemie, which he founded in 1832 and which continued until 1998 as Liebigs Annalen, he publicized both his own work and that of his pupils while also using its pages to criticize the work of other chemists. A giant among 19th-century German chemists, his charismatic power as a teacher and friend was aptly conveyed by his former student A.W. Hofmann (Hofmann, August Wilhelm von): “Each word of his carried instruction, every intonation of his voice bespoke regard; his approval was a mark of honour, and of whatever else we might be proud, our greatest pride of all was having him for our master.”

Liebig was buried in Munich's Südfriedhof Cemetery. Statues were erected in his honour at Darmstadt, Giessen, and Munich. Liebig's former laboratories in Giessen are now the Liebig Museum.