Aldo was born into a fencing family in Livorno, Italy, and both Aldo and his brother Nedo Nadi were fencers from a very young age. They were both taught in the classical Italian school of swordsmanship by their father, Beppe Nadi, and both were already winning competitions by their teenage years. Nedo is generally considered the better of the two. His opponents include the famous French fencer, Lucien Gaudin.
In 1920, at the age of 21, Nadi won gold medals at the Olympics in team foil, team épée, and team sabre. He also won a silver medal in individual sabre, second only to his brother Nedo Nadi.
At age twenty-four, Nadi engaged in a duel with an Italian fencing critic. Nadi was victorious, wounding his opponent several times, sustaining himself a slight wound.
Nadi emigrated to the United States in 1935. He taught fencing in New York City from 1935-1943. In 1943, his book, On Fencing, was published. Also in 1943, he relocated to Los Angeles, California, where he continued to teach fencing in his own school, in addition to occasionally coaching actors for fencing scenes in films. He even made a film appearance himself in To Have and Have Not (1944).
In 1955, Nadi wrote his autobiography (The Living Sword: A Fencer's Autobiography), which was published 30 years after his death. This was done with the help of editor Lance Lobo, and included an afterword by William M. Gaugler, an Italian school fencing master and one of Nadi's students.