Despite being considered a novice in the event, the 17-year-old won with a time of 58.93 seconds.
It was the nation's fastest time by a high school girl until Saturday. Dalilah Muhmmad of New York ran a 57.09 at the Nike outdoor national championships.
"I would say [that] victory [was] a little more exciting [than winning at the PIAA state meet], because no one ever expected that to happen. At least I know I didn't," Krais said.
Winning that race was one of her many milestones en route to being named The Inquirer's Southeastern Pennsylvania girls' track and field athlete of the year for the second consecutive season.
Krais followed that Penn Relays performance with a personal-best 57.26 to finish second at the Nike meet. In addition to having the nation's second-fastest scholastic time in the 400 hurdles, it is the 16th-fastest overall by an American.
Krais also placed fourth (13.92) in the 100 hurdles at the national scholastic meet.
However, her most notable performance in the 100 hurdles came at last month's PIAA track and field championships at Shippensburg University. Krais won the Class AAA race in a meet-record time of 13.73.
She bettered the 13.88 she posted in the semifinals. Penn Wood's Dawn Burrell had set the previous record of 13.96 in 1991.
Krais also ran a 42.04 to win her third consecutive 300 hurdles title and leaped 18 feet, 73/4 inches to win her second straight long-jump crown.
She ended the two-day state meet by anchoring Methacton's 4x400 relay team, which finished third in 3 minutes, 53.16 seconds.
Contact staff writer Keith Pompey at 610-313-8029