Banerjee, who reached the final by beating France's Sascha Gueymard Wayenburg in the semi-finals, is the first American junior champion at Wimbledon since Reilly Opelka in 2015, and 12th overall.
Novak Djokovic has a chance to win a record-equaling 20th Grand Slam title when he plays Matteo Berrettini in the Wimbledon final.
Viktor broke back in the next game, raising hopes of another fight back. But Samir did not give him that opportunity and sealed victory by winning the next game.
The 17-year-old right-hander from New Jersey beat Sascha 7-6(3), 4-6, 6-2 in two minutes short of two hours in their semi-final encounter.
