Wallis scored in the 21st and 47th minutes and North Carolina defender Cat Reddick tallied in the 63rd for the Americans, who went 3-1-0 in Finland to claim their fifth championship in six years at the world's premier U-21 women's tournament.
Martina Mueller scored Germany's goal seven minutes into the second half.
Wallis powerfully headed home a cross from Penn State's Joanna Lohman for the first goal, then extended the U.S. lead at the start of the second half by knocking in a rebound after German goalkeeper Ursula Holl blocked a shot by Santa Clara's Aly Wagner.
Mueller halved the deficit with a 25-yard blast to the upper left after UCLA's Nandi Pryce cleared a German corner kick. The Germans, held without a shot in the first half, applied heavy pressure until Reddick redirected a shot by Penn State's Christie Welsh from a corner kick to make it 3-1.
Claudia Pohlers, like Mueller a key player for Germany's full national team, entered the match with five tournament goals but couldn't beat U.S. keeper Hope Solo.
Holl had an outstanding match for the Germans, making several diving stops among her 10 saves.
Lohman was the Americans' most outstanding player in the tournament.
July 28 at Turku, Finland
United States 3 Germany 1. Goals: Wallis 21, 47, Reddick 63; Mueller 52.
United States -- Solo; Marquand, LePeilbet, Pryce, Reddick; Hawkins, Lohman, Wagner; Zepeda (Lloyd, 69), Ramsey (Welsh, 53), Wallis (Crumpton, 82).
Germany -- Holl; Zietz, Rech, Hagedorn; Wilmes (Hartmann, 14; Derda, 46), Meier, Machalett, Omilade (Boschert, 75), Kliehm; Mueller, Pohlers.
Yellow cards: USA -- Solo 54; Germany -- Mueller 78.
Referee: Maria Persson (Sweden).
Third: Norway 5 Denmark 1.
Fifth: Sweden 3 Finland 1.
Seventh: Iceland 4 Greece national team 1.