He was general manager of La Scala in Milan, Italy, from 1898 to 1908 and later the Metropolitan Opera in New York City from 1908 to 1935.

Under his leadership the Metropolitan enjoyed a prolonged era of artistic innovation and musical excellence.

The Viennese composer Gustav Mahler also was a Met conductor during Gatti-Casazza's first two seasons and in later years conductors Tullio Serafin and Artur Bodanzky led the company in the Italian and German repertories respectively.

[1] Affectionately called "Gatti" by friends and colleagues, Gatti-Casazza's prodigious artistic and organizational skills attracted the best singers and conductors to the Metropolitan, and, on 10 December 1910, hosted its first World premiere, La Fanciulla del West by Giacomo Puccini.

Many noted singers of the era appeared at the Met under Gatti-Casazza's leadership, including Rosa Ponselle, Emmy Destinn, Frances Alda, Amelita Galli-Curci, Maria Jeritza, Lily Pons; Enrico Caruso, Jacques Urlus, Giovanni Martinelli, Beniamino Gigli, Feodor Chaliapin, Titta Ruffo, Giacomo Lauri-Volpi, and Lauritz Melchior.