Redi, who was born at Arezzo in Italy, studied medicine and philosophy at the University of Pisa, graduating in 1647. He was employed as personal physician to Ferdinand II and Cosimo III, both grand dukes of Tuscany. Intellectually, Redi displayed a variety of talents, being a noted poet, linguist, literary scholar, and student of dialect. On the scientific side, he laid the foundations of helminthology (the study of parasitic worms) and also investigated insect reproduction.

As a biologist he is best known for his experiments to test the theory of spontaneous generation. These were planned to explore the idea, put forward by William Harvey, that flies and similar vermin do not arise spontaneously but develop from eggs too small to be seen. Redi prepared eight flasks of various meats, with half left open to the air and half sealed. Maggots were found only in the unsealed flasks where flies had been able to enter and lay their eggs. That this effect was not due to the presence or absence of fresh air was shown by a second experiment in which half the flasks were covered with fine gauze. Again, no maggots developed in these. This was one of the earliest examples of a biological experiment planned with proper controls. Redi still believed, however, that spontaneous generation occurred in such animals as intestinal worms and gall flies, and it was not until the time of Louis Pasteur that the spontaneous-generation theory was finally discredited.