From the American Golfer, August 1913:
The Siwanoy Country Club, of Mount Vernon, has recently acquired a tract of 110 acres about three- quarters of a mile from the present course, and Donald Ross has laid out 18 holes which, in his estimation, offer promise of being one of the finest lay- outs in the Metropolitan district. On the property and over-looking Long Island Sound, is situated a large manor house, which will be converted into an up-to-date clubhouse.
The property is an historic one. Originally owned by a Tory, it was confiscated by the State during the Revolutionary War and presented to John Paulding for his part in the cap- ture of Major Andre.
I have also seen a Carter's ad from April 1914 indicating that the course was seeded in September 1913.