The Promising Young Woman star, 38, revealed she had given birth in an interview with Vogue magazine for its November cover story.
Mulligan already shares a daughter Evelyn and son Wilfred with her 36-year-old musician husband Mumford, who she married in 2012.
In the article, the interviewer noted that Mulligan entered “a London coffee shop with a bemused gesture at the yellow diaper-leak stain streaked across her blouse”.
While carrying the infant, Mulligan then said to her baby: “I’m going to change you, and then we’re going to find Mum another shirt.”
The actress has starred in a host of critically acclaimed films including Emerald Fennell’s Promising Young Women, which earned her an Oscar nomination, Baz Luhrmann’s The Great Gatsby and 2022’s She Said – which sees her play one of the New York Times reporters that exposed Harvey Weinstein’s history of abuse to women.
Her latest project Maestro sees her portray actress Felicia Montealegre, the wife of esteemed conductor and composer Leonard Bernstein, who is played by Bradley Cooper.
Reflecting on her life with Mumford, who is the lead singer of folk band Mumford & Sons, she said: “It’s very normal.”
She added that it consists of “school runs, Sunday lunches” but admitted that their careers can cause some “tricky logistics”.
The actress, who keeps out of the limelight aside from her movies, also revealed that she feels her life is like travelling in the Doctor Who Tardis at times.
She said: “Most days I’m just me, I don’t feel famous when I’m out in the country, most of my friends aren’t in the industry, I have this nice, very regular life.
“And then every once in a while I step into a magic phone box, and — whoosh — I come out the other side in a designer gown and there are lights flashing everywhere.”
Her upcoming movie Maestro also features a parallel set up to Mulligan’s life in that it has an actress wife and a musician husband at its core.
However, the actress feels it was “different” for Montealegre as “everything revolved around Lenny”.
“There was a lot of ‘What if?’ with her character…. What if she hadn’t given up acting?” she added.
“When I listened to tapes of her being interviewed, it seemed like she wasn’t sure how far she’d have gone — that she felt like, maybe she didn’t have it in her to be great. But on the other hand, she never got the chance to find out.”
Mulligan feels the movie is not a biopic, but more of a story about a “very complicated marriage”.
The full interview is available in Vogue.