Merriam-Webster's dictionary has 1887 for "parkway."
8 February 1869, Brooklyn Daily Eagle, pg. 6:
...in addition to this the full development of the great south-eastern parkway through the heart of the Ninth Ward, which is now in the hands of commissioners of estimate and assessment, will require this splendid improvement to be carried through to the plaza east of the reservoir, and will still further lessen the capabilities of this portion of the Park.
5 October 1869, New York Times, pg. 2:
ANOTHER BROOKLYN BOULEVARD.
Meeting of the Commissioners of Estimate
and Assessment for the Southeastern
Parkway - Description of the Plan.
The Commissioners of Estimate and Assessment for the property affected by the laying out of the Southeastern Parkway or Boulevard leading from Prospect Park toward East New-York, met at the office of the Prospect Park Commissioners Yesterday, Mr. NICHOLAS VAN BRUNT in the chair. There was an unusual array of legal gentlemen in attendance, representing various property owners along the line of the proposed Parkway, and some objections were filed to the awards of the Commissioners on the ground of insufficiency of amount. The Parkway, according to the plan at present contemplated, provides for one of the most magnificent Boulevards in the world. It will run along the present line of Sackett-street, from the southeasterly side of Prospect Park, immediately in the rear of the reservoir, toward East New-York, but stopping for the present at the City line, a distance of two and a half miles.