Shantia Dennis, 26, was arrested after police said undercover agents purchased heroin from the woman while she was working at the McDonald's outlet in Pittsburgh, Pennsylvania.
Authorities received information that heroin was being sold by an employee at the restaurant, Mike Manko, a spokesman for Allegheny County District Attorney Stephen Zappala, said.
Investigators said customers looking for heroin were told to go through the drive-through and say "I'd like to order a toy."
The customer would then be told to drive to the first window, where they would hand over their money and get a Happy Meal box containing heroin in exchange, WPXI-TV reported.
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Once the controlled buy was executed on Wednesday, detectives arrested Dennis.
After the arrest, 10 stamp bags of heroin were recovered inside the Happy Meal box and another 50 bags were recovered from Dennis, Manko said.
"No I was not," Dennis told Channel 11, on being asked if she was selling heroin at McDonald's.
Manko said it's unclear how long the process was going on or how many transactions have taken place.
Investigators say Dennis' alleged actions were even more dangerous because she was using a kid's meal to push the deadly product.