"It's a girl," Mulligan revealed on Friday's The Graham Norton Show, adding that the baby was born three weeks ago.
Mulligan, 30, did not share any more details. The actress kept her pregnancy under wraps, quietly debuting her baby bump at the Tony Awards in June. News that she had given birth only broke last week after she showed off her post-baby body at an event for her new film Suffragette.
Mulligan and Mumford, 28, who were childhood pen pals, wed on a farm in 2012.