Little Exton weighed 7 pounds 5 ounces at birth and measured 20 inches long. It's the first child for producer Susan, 38. "Sherlock Holmes" star Downey, 46, has an older son, 18-year-old Indio, with ex-wife Deborah Falconer.
The actor, who took Susan on a romantic Hawaiian getaway back in November, revealed his baby-to-be's gender back in December.
Downey joked about second-time fatherhood to Jay Leno on "The Tonight Show" late last year. "I don't wanna be presumptuous," he said. "But I think actually [the pregnancy] has been tougher on me [than Susan]! ... Just the hormones on and the mood stuff and the nausea and the whole thing."