Irmelin Indenbirken, who divorced the 38-year-old actor's father George when he was a baby, showed off her beau of one year, David, at an Independence Day party, reported New York Post online.
A guest who asked David about his romance with Irmelin, said, "He said Leo and his (own) daughter, the wife of a Black Keys band member, set them up on a blind date, and they've been together ever since."
"The two danced next to Leo... And partied away until the wee hours."
The holiday bash was a double date for the mother-son duo.
Other guests at the lavish party included Charlize Theron, Owen Wilson, Mel Gibson, Woody Harrelson and George Clooney's girlfriend, Stacy Kiebler.