"It's a girl," Mulligan, 30, gushed.
Us Weekly confirmed last week that the Suffragette star and the Mumford & Sons frontman welcomed their first child together.
"The baby is healthy," an insider told Us at the time. "They have been loving being parents and their friends and family are thrilled."
Mulligan didn't go into detail about her newborn daughter on Monday, but she did show off her post-baby body. The new mom looked slim in a black plunging blazer and trousers.
As previously reported, Mulligan stepped out for her first red carpet since giving birth at the British Film Institute's London Film Festival on Oct. 7, to promote her new drama. Suffragette also stars Meryl Streep, Helena Bonham Carter, and Anne-Marie Duff.
Mumford popped the question in August 2011, just five months after dating. The couple tied the knot in April 2012 in Somerset, England.
Suffragette hits theaters on Friday, Oct. 23.
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