Amadio, a center, was selected by the Los Angeles Kings in the third round (No. 90) of the 2014 NHL Draft. He established himself as a prolific scorer at the bantam and midget levels before spending four seasons with Brampton and North Bay of the Ontario Hockey League. His best season in junior hockey came in 2015-16, when he had 98 points (50 goals, 48 assists) in 68 games with Brampton.
Amadio signed a three-year contract with the Kings on March 13, 2016. He joined Ontario of the American Hockey League for the Calder Cup Playoffs and was recalled by Los Angeles on Oct. 26, 2017 after beginning the season in the AHL. He scored his first NHL goal during his fourth game with the Kings, a 5-3 win against the Toronto Maple Leafs on Nov. 2, 2017, and finished his first season with eight points (four goals, four assists) in 37 games.