Harrison, a former youth team player at Manchester United before moving to the United States aged 14, has impressed in Major League Soccer since making his NYCFC debut in May 2016.
"I was thrilled when I found out; I was really speechless," explained Harrison, speaking to NYCFC's official website.
"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 traveling over and doing the best I can. It's an honor to represent my country."