The 16-year-old got his chance in the Carabao Cup third round against Watford.
Branthwaite starred in a 2-1 win over the lower division side, with the defender playing alongside several youth and backup players.
Senior stars Jeremy Doku and Matheus Nunes got the two goals in the contest for City.
Post-game, Branthwaite said: “It’s a dream come true to be fair. I’ve just been preparing for this moment my whole life. To do it today was just unbelievable for both me and my family.
“They were in the crowd, I don’t even know where they were but they were there supporting. I think they’ll be proud more than anything.
“Obviously, my dad majority just taking me to training. And my mum has always been there cheering and supporting on the sidelines.
“They’ll just be buzzing. I just took it in my stride.”