Ewan's daughter Clara McGregor confirmed the big news by posting sweet snaps on social media. The actors, who met and fell in love while filming TV drama "Fargo" together in 2017, never even announced Winstead's pregnancy.
"Welcome to the world little brother congratulations to my Dad & Mary - this is the greatest gift," Clara wrote beneath a shot of herself cradling the newborn in her arms.
The baby is Ewan's first son - he and his ex-wife, Eve Mavrakis, share three biological daughters, Clara, Esther and Jamyan, and they adopted a baby girl, called Anouk, a decade ago.
He and Eve split in 2017 and their divorce was finalized in August (2020).
Clara previously was very outspoken about her father's new romance after a fan tagged a photo of Ewan and Mary sharing a kiss at a Vanity Fair Oscars party, calling the actress "the most beautiful and talented woman on earth."
"Most beautiful and talented woman on earth???" she responded. "Oh man y'all are delusional. The girl is a piece of trash."
Several months later, Ewan's daughter acknowledged the immaturity of her Instagram post, adding, "I was angry and upset."
"There had been a lot building up to it and a lot to deal with - not to make excuses or anything - but, yeah, it wasn't my finest moment," she explained.
"I said how I felt and I didn't want to apologize for it. It wasn't the right way to go about things, but it's a hard thing to wrap your head around when you feel you had this idea of what the family unit is and then to have that shift. It's very weird."