The 20-year-old was born in Hamburg but raised in England after his parents relocated while he was still very young.
Nmecha has represented England at every youth level between Under-16 and Under-21, but he made his debut for the Germany Under-21 side this week.
The City starlet came on as a second-half substitute as Germany secured a 2-1 win over the Three Lions at Bournemouth's Vitality Stadium on Tuesday.
Speaking after the match, Nmecha revealed he is switching allegiances permanently, after giving the issue a great deal of thought.
He stated: "I chose Germany, where I was born, I've been thinking a long time about what I really want, that was the right decision, and it's final.
"I learned 10 minutes before the game that I'm allowed to play, it was just pure joy, and the coach was happy with me."
Nmecha's international U-turn comes just four months after he scored a goal for England's U20 side in a 2-0 win over Germany, in a stunning 30-minute cameo which also included an assist.
The attacker went on to explain why he has ultimately chosen to pledge his international future to Germany over England, crediting U21 coach Stefan Kuntz for talking him round.
He also expressed his desire to earn a spot in Kuntz's squad for the European Under-21 Championship this summer, which are taking place in Italy and San Marino.
"The coach was at my house and talked about the team spirit, he convinced me," Nmecha added.
"I feel really good in this team and hopefully I will play in the European Championship and then I want to win there as well."
At club level, Nmecha has spent the 2018-19 campaign on loan at Championship side Preston North End, but he is due to return to the Etihad Stadium this summer.
The young attacker has contributed two goals and four assists in 33 matches for Preston, helping them to launch a play-off bid.