Human rights lawyer Clooney was photographed walking into the Manhattan hotel in a bright red jumpsuit and gold heels with a black coat over her shoulder.
Canadian stylist Jessica Mulroney, whose daughter Ivy served as a bridesmaid at Meghan Markle and Prince Harry’s wedding, arrived with a smile as the snow fell.
Abigail Spencer, who starred in the US drama Suits with Meghan, has also been spotted arriving at the hotel.
Other celebrities attending include harpist Erin Hill, fashion designer Misha Nonoo and television presenter Gayle King, all close friends of the duchess.
The Mark Hotel overlooks Central Park and boasts 141 spacious rooms and suites designed by French designer Jacques Grange.
The mother-to-be is expected to depart New York on Wednesday evening as she is due to join Harry on their official three-day tour of Morocco on Saturday.
Meghan is preparing to give birth in late April or early May.
Kensington Palace has declined to comment on the break in New York.