Lyn Richie, 38, said she had taken her husband, daughter Ellyn, six, and stepdaughter Katy, 12, to the fast-food restaurant in Coleraine, Londonderry.
But the mum, who vows never to return to the restaurant again, said their meal was ruined after Ellyn found the nail sitting on top of the burger patty.
Richie said, according to the Mail Online: "'At first, I thought it was an onion or a false nail, but was horrified when staff confirmed it was human."
Richie said her daughter pulled the burger apart before eating it because she doesn't like bread, and was very glad she did.
The mother claimed staff members offered a refund of £16.77 for the entire order after confirming the nail was in fact a human one.
Richie said her daughter wasn't really aware what was going on, but was "gutted" that her burger had been taken away.
However when she knew the origin of the nail, she told her mum she felt sick thinking about it.
Lyn Richie and her daughter, Ellyn
The mother has lodged an official complaint with McDonald’s and the environmental health department at Coleraine Borough Council. They are conducting a joint investigation.
Richie said, according to the NY Daily News: "It just sickens me - it seriously makes you questions their hygiene practices."
As a childminder, she said she regularly took children to McDonald's for a treat, but can no longer trust them.
A McDonald’s spokesperson told HuffPost UK Parents: "We are sorry for Ms Richie’s experience when visiting our restaurant.
"We have launched an investigation following the reporting of this incident and our Customer Service department will be in touch with Ms Richie to keep her updated.
"Food safety and quality are our highest priority and both our restaurants and our suppliers place great emphasis on quality control, following rigorous standards in order to avoid imperfections in our products.”