Mulligan, 37, shares two children – daughter Evelyn, seven, and son Wilfred, five – with the Mumford and Sons frontman.
They began dating in 2011, and got engaged after five months. Mulligan and Mumford, 35, tied the knot on a farm in Somerset, England, with 200 guests in attendance – including Colin Firth and Sienna Miller.
A representative for the actor told Peoplethat the couple were expecting their third child together, in a statement on Saturday (14 January).
Mulligan was most recently seen in She Said, in which she plays one of the two New York Times journalists who exposed the sexual abuse allegations against Harvey Weinstein.
She plays the role of Megan Twohey who, along with Jodi Kantor (played by Zoe Kazan), helped bring down one of Hollywood’s most powerful men.
The British actor has previously spoken about suffering postnatal depression, following the birth of Evelyn.
However, returning to work to promote her 2015 film Suffragette helped her heal. she told Variety.
“It was either cancel the whole thing or just get on and do it. And that – and a combination of lots of other things, and help and support from everyone around me – was my light,” Mulligan said.
She also said her experiences helped her connect with Twohey, whose struggle with postpartum depression is depicted in She Said.
“Megan and I talked about that, and we both shared what we had both been through, like so many women have been through,” Mulligan shared at the time.