Nix is replacing Cathy Russell, who was nominated last month as the ambassador-at-large for Global Women’s Issues.

President Obama praised Russell as "a longtime advocate for women, for justice, for fairness" in remarks announcing her departure.

"She’s worked tirelessly alongside Michelle and Jill to make sure that our military families get every single benefit and bit of assistance that they so richly deserve and have earned," Obama said. "I’m certain that Jill will miss Cathy, but I know she joins me in saying that we could not be prouder of Cathy’s hard work and her advocacy."