Jacob received his M.D. degree (1947) and a doctorate in science (1954) from the Faculty of Medicine and the Faculty of Science, respectively, in Paris. Most of the work of Jacob, Lwoff, and Monod was carried out at the Pasteur Institute (Paris), which Jacob had joined as a research assistant in 1950. In 1960 he became head of the department of cellular genetics at the institute, and from 1965 he was also professor of cellular genetics at the Collège de France. In 1977 he became a member of the Academy of Sciences.

With a coworker at the Pasteur Institute, Jacob discovered that the genes (gene) of a bacterium are arranged linearly in a ring and that the ring can be broken at almost any point. In 1958 Monod and Jacob began to collaborate in studies of the regulation of bacterial enzyme synthesis. One of their first major contributions was the discovery of regulator genes, so called because they control the activities of structural genes. The latter, in turn, not only transmit hereditary characteristics but also serve in the production of enzymes, other proteins, and ribonucleic acid (RNA). (See operon.)

Jacob and Monod also proposed the existence of an RNA messenger, a partial copy of the gene substance deoxyribonucleic acid (DNA), that carries genetic information to other parts of the cell. They also found that in a normal cell the balance between regulator and structural genes enables the cell to adapt to varying conditions. An interruption in this balance, however, can stimulate the production of new enzymes that can prove either beneficial or destructive to the cell. In addition to his research activities, Jacob has also written important books in the history and philosophy of the life sciences, including The Logic of Life (French edition 1970; English translation 1973).