The small, private gathering included Nicky Hilton, who is the godmother to Johnson's young daughter Ava.
No other details were available.
The Johnson & Johnson heiress, who was recently engaged to reality star Tila Tequila, was found dead in her Los Angeles home on Jan. 4. She suffered from diabetes, but no cause of death has yet been determined.
Family spokesman Jesse Derris last week thanked well-wishers for their support. "Your thoughts and prayers are deeply appreciated," he said.
Donations in Johnson's memory can be made to the Casey Johnson Memorial at the Juvenile Diabetes Research Foundation.