He tipped the staff $3,000 on a $39 check at the Brief Encounter restaurant in Bellevue.
"At first, they didn't know who it was," diner owner Melanie Bard said of her mystified employees.
The 12-person staff initially thought it was a mistake, but the back of the receipt spelled out a story from the big tipper:
"He's a great customer when he comes in," Bard said, adding that he has been coming in for the past eight years.
Clark said he wanted to “do something in appreciation of my mother, who's not with us anymore, and because of the Christmas season.”
He remembers tagging along with his mother as she worked in the food service industry to provide for him and his three siblings, putting all four of them through college.
"I saw how hard my mom worked for people who weren't always appreciative," Clark said.
Clark left the tip with Julie Welsand, his waitress at the diner Saturday, a spokeswoman for his company said
The staff will split the tip evenly, $250 each, as Clark requested, which Bard said helps them out the gift-giving season.
Neither Welsand nor the other employees was available to speak with ABC News.
Clark stressed that this is isn't about the amount of money he gave, but instead about the sentiment behind it.
"You don't have to give money. You can write appreciative notes and say thank you," he said of wanting to inspire a tipping movement this holiday season. "It would go a long way to nourish all of us."