On 11 July 1899, Giovanni Agnelli was part of the group of founding members of FIAT, Fabbrica Italiana di Automobili Torino. The first Fiat plant opened in 1900[8] with 35 staff making 24 cars. Known from the beginning for the talent and creativity of its engineering staff, by 1903 Fiat made a small profit and produced 135 cars; this grew to 1,149 cars by 1906. The company then went public selling shares via the Milan stock exchange.

Agnelli led the company until his death in 1945, while Vittorio Valletta administered the firm's daily activities. Its first car, the 3 ½ CV (of which only 24 copies were built, all bodied by Alessio of Turin)  was based on a design purchased from Ceirano GB & C and had a 697 cc (42.5 cu in) boxer twin engine. In 1903, Fiat produced its first truck. In 1908, the first Fiat was exported to the US. That same year, the first Fiat aircraft engine was produced. Also around the same time, Fiat taxis became popular in Europe.