McKennie, 22, secured a move from German side Schalke 04 to the defending Italian champions last month on a one-year loan deal with an option to permanently join at the end of the 2020-21 season.
But it took no time for the Texas native to integrate himself into manager Andrea Pirlo's XI, where he will offer an unrelenting engine and leadership to a midfield which also includes Arthur, Rodrigo Bentancur, Aaron Ramsey, and Adrien Rabiot.
Italy legend Pirlo, making his managerial debut, saw Dejan Kulusevski, signed from Atalanta, scored in the 13th minute with only his sixth touch of the ball in a Juventus shirt before defender Leonardo Bonucci and Cristiano Ronaldo added two more in the last 15 minutes.
Juventus kick off the season with 3-0 win vs. Sampdoria
Defending champions Juventus start their title defense with an emphatic 3-0 win over Sampdoria. Watch Serie A on ESPN+.
McKennie impressed in the match, going all 90 minutes and nearly scored twice in Sunday's victory. He is the latest American player to play in the Italian top flight, after Alexi Lalas (Padova 1994-96), Michael Bradley (Chievo, Roma 2011-14), Joshua Perez (Fiorentina 2016-17), Armando Frigo (Fiorentina 1939-42) and Alfonso Negro (Fiorentina, Napoli 1934-38).
Another U.S. international, Oguchi Onyewu, did join AC Milan in 2009 but didn't make a single Serie A appearance. Giuseppe Rossi, who has played for several Serie A sides, was born in New Jersey but represented Italy at the international level.
Juventus are aiming for their 10th consecutive Italian title this term, as well as trying to win their first UEFA Champions League title since 1996.