Harrison, who spent time in the Manchester United youth ranks before departing for the United States at the age of 14, has tallied 10 goals an six assists in 32 games for New York this term.
Speaking to his club side's official website, the Stoke-born midfielder said: "I was thrilled when I found out; I was really speechless. It's an exciting opportunity and one I'm very grateful for. I never thought this would happen when I moved out here when I was 14. It's a special moment and I'm going to cherish it.
"Every year this league is growing and you only have to watch the games to see that it's getting better and better every year. I'm so excited to be travelling over and doing the best I can. It's an honour to represent my country."
NYCFC manager Patrick Vieira added: "It's really good news for Jack, it's really good news for the football club and of course I'm really pleased for him because he's improved a lot and he loves the game.
"I just want him to enjoy it - it will be a great, positive experience to be surrounded by players who play in the Premier League and I just want him to show how good he is because if the manager called him, it means he deserves it."
The Young Lions drew 1-1 with the Netherlands and beat Latvia 3-0 in their opening two Euro 2019 qualifying fixtures.