Craig Barritt/WireImage
People Staff
September 11, 2011 08:05 PM

This U.S. marshal now has her own posse.

Mary McCormack, who hunts down witnesses on In Plain Sight, delivered her third daughter, Lillian, on Saturday morning, Sept. 10, her rep confirms to PEOPLE.

Lillian weighed in at 6 lbs., 15 oz. and is 20 inches long.

The actress, 42, and husband Michael Morris s new arrival joins big sisters Rose, 4, and Margaret, 7.

McCormack – who gave PEOPLE a tour of her trailer on-set – had her pregnancy written into the storyline of her character Marshal Mary Shannon. – Julie Jordan

PHOTOS: Who’s Due Next?

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