The 19-year-old midfielder rose to head home into the bottom left corner in the 62nd minute after a fine cross from Julian Brandt.
Mainz, who suffered their first defeat of the campaign at BayArena, had the ball in the net four minutes into the second half but Robin Quaison's effort was disallowed for handball.
In the day's other game, RB Leipzig stretched their unbeaten league run to three games by coming from behind to draw 1-1 at Eintracht Frankfurt.
Emil Forsberg earned a point for the visitors at Waldstadion, converting a 54th-minute penalty down the centre of Kevin Trapp's goal after Frankfurt midfielder Filip Kostic was penalised for handball.
Former Manchester City midfielder Gelson Fernandes tapped home a rebound to give the hosts a 26th-minute opener, while Leipzig were denied a 12th-minute opener when Timo Werner's effort was ruled out by an offside flag.
