Ermenegildo Zegna was born in 1892, the youngest of the ten children of Michelangelo "Angelo" Zegna (1859–1923), a watchmaker from Trivero, in the Biellese Alps, who had opened a textile factory in nearby Flecchia. In 1910, aged 18, Ermenegildo founded the Zegna & Giardino wool mill in Trivero with two of his brothers and a fourth partner, Costanzo Giardino Vitri. After Vitri and one brother left early, Ermenegildo led the business from the start, joined by his brother Mario until the early 1940s.
His aim was to make the finest fabrics, so he bought wool directly from Mongolia, Australia and South Africa and machinery from England. The mill flourished between the wars, with more than 700 workers in the late 1920s and over 1,000 by the late 1930s; in 1938 it began exporting to the United States through Zegna Woollens Corporation in New York. In 1942 his sons Aldo (born 1920) and Angelo (born 1924) joined, and the firm became Ermenegildo Zegna and Sons, with 1,400 staff by the mid-1950s.
From the 1930s he promoted welfare projects for the Trivero community and built the Panoramica Zegna, provincial road 232, through the company's home territory; more than 500,000 conifers were planted along it in the late 1930s, the origin of the 100 km² Oasi Zegna nature area established in 1993. The road inspired the logo adopted when the group was listed in 2021. His sons took over in the mid-1960s. Casadi has not verified his date of death.