The First Lady had picked out a vibrant patterned dress to welcome the Princess and her husband Prince Albert. Charlene went for an even bolder choice in a black all in one with strappy sandals.
The Monegasque royals feel very much at home in the States because of Albert's links to America through his late mother, Princess Grace.
With the UN assembly underway, Michelle was hosting a lunch for 49 wives of visiting UN dignitaries and Valerie Trierweiler, the partner of French president Francois Hollande.
Michelle spoke about the need to improve the lives of women and girls around the world, telling guests to "share ideas for how we can all give our children the bright futures they deserve.
"Don't be shy, make sure that you talk to the ladies at your table about the great work that you are doing in your countries because you are all doing some wonderfully powerful work," she said.
The reception took place at Harlem's Studio Museum, a gallery for modern and contemporary black art.
"There's a reason why I wanted to bring you all to Harlem today," said the First Lady. "And that is because this community… is infused with a kind of energy and passion that is quintessentially American, but that has also touched so many people around the world."