Sources told Us Weekly the couple are set to get married after two years of dating. They live together in New York City.
It will be the first marriage for Olsen, the second for Sarkozy, who has two children, Julien, 12, and Margo, 10.
"Mary-Kate has made him a better father. She remembers all the kids' special events," a source previously told Us. "She thinks he's a great dad."
"Mary-Kate's family thinks Olivier is the best thing to happen to her," another source said. "They love him."