The King was born in Valencia, Venezuela. He was first spotted by Luis Fuenmayor, a part-time Mariners scout who saw him pitching at age 14 in a tournament near Maracaibo, Venezuela. Fuenmayor recommended Hernández to fellow scouts Pedro Avila and Emilio Carrasquel, who were impressed with the youngster who could already throw 90 mph. The Seattle Mariners signed Hernandez as soon as he turned 16, in accordance with MLB rules.
After graduating from high school, Hernández finally agreed to his first professional contract. Mariners director of international operations Bob Engle signed Hernández as a nondrafted free agent on July 4, 2002. Hernández received a large signing bonus of $710,000, although he said the Mariners were not the highest bidder. Other teams trying to sign him included the New York Yankees and the Atlanta Braves, with both teams reportedly offering more money than Seattle.
The following year, Felix came to the United States and began pitching in the Mariners’ minor league system. In 2003, Hernández tore through Class-A with a 7–2 mark in Everett and Wisconsin. Returning to his native Venezuela to pitch in the winter league there, he held his own at 17 years of age against competition that included established major league players.