The original historic district was designated in 1971 (that's the gray shaded part of the map above) by the Landmarks Preservation Commission. In 1996, the boundaries of the district were expanded (see the blue shaded parts of the map). (Note: The National Register historic boundaries were expanded, but the New York City historic district boundaries have not yet been expanded)
What made this area worth designation as a historic district? Directly from the Designation Report from 1971:
The Mount Morris Park Historic District includes a remarkably interesting cross-section of turn of the century townhouses and churches. These buildings represent many styles of architecture spanning a period of over four decades. The churches and individual residences are among the finest in the City. The quality of design and workmanship of these buildings establishes the exceptional character of the District. Outstanding among them is the row of townhouses on Mount Morris Park West. The survival of a substantially unbroken row of handsome residences and churches facing a park is, in itself, rare in Manhattan.
The proposed Mount Morris Park Historic District is a fine residential area which has maintained its attractive late 19th century architectural character remarkably well. This part of the City was largely built up during the latter part of the 19th century, with rows of unusually handsome townhouses and several notable churches. Some of the most stately residences are to be found along Mount Morris Park West.And it really is true that the 19th century architectural character has been maintained - heck, if you walk down some of those blocks (and pretend the cars aren't lining the street), you really can imagine what the area looked like 100+ years ago.
The buildings in this District represent many architectural styles including the Romanesque Revival, the French neo-Grec, the Queen Anne and a number of structures designed in the classical and Renaissance traditions popularized by the World's Columbian Exposition of 1893 in Chicago.
Anybody have firsthand information about the historic district formation and why the boundaries are what they are?