People.com snapped a photo of Williams in Manhattan Monday.
Williams and Ledger, who reportedly fell in love on the set of their 2005 film "Brokeback Mountain," are the parents of 2-year-old Matilda.
The family lived together in Brooklyn, N.Y, until September 2007 when Williams and Ledger split up.
Ledger died of an accidental, lethal mix of prescription drugs on Jan. 22. He was 28.