"It was very tough today, especially with the wind," said the freshly crowned European Games champion.
The 25-year-old Belgian Champion beat compatriot Kelly Druyts (Topsport Vlaanderen Pro-Duo) to the line, at the end of the 124km race.
King's performance, against most of the top riders in the British domestic peloton, confirms that the 24-year-old has recovered from her horrific crash of last November.
The Philly Classic sees the World Cup return to the United States for the first time since its previous incarnation.
The Japanese Champion managed to bridge across to a late break from Gracie Elvin (Orica-AIS) and Ellen van Dijk (Boels-Dolmans) but was unable to compete with the two faster sprinters on the slightly uphill finish.
The Netherlands took the first world title of the evening in the women's Scratch Race, their first gold medal of these championships. Kirsten Wild won the rainbow jersey. The final title of the night went to Germany's Kristina Vogel
"This is my first World Championships," stated Wiasak, "I have never even won a national championship, so it is fantastic to get a World title.
Russia's Anastasia Voynova took gold after finishing in the bronze medal position last year, while world record holder Anna Meares of Australia retained silver, and defending champion Miriam Welte of Germany dropped to bronze. UCI ©Guy Swarbrick
The 2015 UCI Track Cycling World Championships opened in St-Quentin-en-Yveline, France, on Wednesday with a world record time set, plus a win for the host nation.
The staging of the world championships in the Lee Valley VeloPark at Queen Elizabeth Olympic Park