It also was the third time in their last five games that the Kings were shut out. During that span, the Kings have scored only three goals, all of them by center Anze Kopitar.
"You have to score to win, you can't win nothing to nothing," Kings Coach Darryl Sutter said.
The Kings had 35 shots on goal compared with 13 for Philadelphia (27-23-6), a season low allowed by the Kings. But Flyers goalie Steve Mason earned his third shutout of the season.
"We played good defensively," Philadelphia Coach Craig Berube said. "We talked about playing the Kings. They are a big, strong team. They play good defense, we need to match that."
They did, and twice the Flyers converted their own chances.
Simmonds, traded by the Kings to Philadelphia in 2011 and nicknamed "Wayne Train," scored with 12:12 left in the second period.
It was Simmonds' 18th goal of the season and the 100th of his National Hockey League career.
"L.A. is a big team, they like to grind it out," Simmonds said. "I think we matched them hit for hit today."
Giroux scored his 17th goal of the season with 1:58 left in the third period during a Flyers power play. He took a pass from Scott Hartnell and fired the puck past Quick from close range.
"It doesn't matter how we win the games, we've just got to find ways to win," a frustrated Quick said. "We've got to be better as a team."
With the Kings unable to score, Quick has had little margin for error. But, Sutter said, "I don't think it puts pressure on Quick. He's going to play the same way every night."
The Kings meet the Columbus Blue Jackets on Thursday and don't play again until Feb. 26, when they travel to Colorado to face the Avalanche.