Billionaire Leon Prater’s Mansion is actually a composite of three different New York locations (four, if you count his grand helicopter estate, which doesn’t show up until the 7th episode). •    The exterior of the house, •    The gated driveway and parking area, and •    This interior of the mansion were all filmed at different locations around New York.

Q. Where is it supposed to be on the show?

A.  In Manhattan.

Q. When did we see it on the show?

A. We see this location in Episode 4 of Season 1 of Resurrection.
 
After Dexter decides to impersonate the deceased Dark Passenger killer, and accept his invitation to a mysterious meeting, He is picked up in Central Park  by Prater's right-hand man, Charley, and is driven past the front of the mansion, and into an impressive motor court, where she escorts Dexter into an elevator that takes them inside Leon Prater's mansion.

It's there that Dexter has his first meeting with billionaire Leon Prater, who turns out to be 4 ft, 5 inch tall eccentric (played by actor Peter Dinklage), dressed in an impeccable white suit. He greets Dexter warmly, believing him to be brutal serial killer Ron Schmidt, and it soon becomes obvious that Prater has a keen interest in serial killers.

Dexter learns that Prater hosts regular dinner parties for those murderers, offering them a “safe space” at his table, in return for their company and the occasional “trophy” taken from their victims.

Prater eventually takes Dexter inside a secure vault, where he has created a bizarre museum featuring souvenirs from the world's most infamous serial killers, including a hammer from the Trinity Killer, and Dexter's blood slides from the Bay Harbor Butcher.

It's also here that Dexter attends his first dinner party and is introduced to a quartet of infamous killers, including the “Gemini Killer”, “Lady Vengeance”, the “Tattoo Collector”, and “Rapunzel”. 

Q. What is it actually in real life?

A. An historic mansion, once part of a large estate.

 

Q. Where can I find it in real life?

A. Despite the claim on the show that this is a Manhattan mansion that covers an entire city block, this villa, used for the interiors of Prater's home, is actually located outside of New York City, on Long Island's Gold Coast, where many wealthy men of the Gilded Age built impressive estates.

( And the other mansion briefly used for the exterior up the home, is in Manhattan, but doesn't come close to covering its entire block. )     

In  real life, the scenes of Dexter meeting Leon Prater were shot inside the “Villa Carola” (the Guggenheim Mansion), located at 50 Astor Lane, on Long Island, New York.

 

 
The historic home is now surrounded by a country club called “The Village Club of Sands Point”, with the address of 1 Thayer Lane, Sands Point, NY. It originally belonged to members of the Guggenheim family, whose patriarch, a Swiss immigrant named Meyer Guggenheim, built a business empire in the late 19th century, initially in Colorado silver mines and later expanding into copper and lead, ultimately establishing the American Smelting and Refining Company, which became a dominant force in the mining industry. 
 
At one point, the Guggenheim brothers controlled mining empires spanning multiple continents, with holdings in gold, tin, and other precious metals. Today, Guggenheim Partners is a modern financial firm that manages over $300 billion in assets

( This mansion, in Italian Renaissance style, is separate from the castle-like 'Hempstead House', another home owned by Guggenheim, which also shares the same Country Club property. ) 
 
Here is what the Village Club website has to say about the history of the mansion :

“ Welcome to 'Villa Corola', originally the estate of Isaac Guggenheim and later known as the IBM Country Club. 
 
Isaac started building the Manor House in 1916, completing it in 1918. 
 
After his death in 1922, his brother, Solomon R. Guggenheim purchased the property in 1924. Solomon, a key figure in the Guggenheim family's fortune, used his wealth for art, most notably founding the Guggenheim Museum in New York. 
 
The estate was redesigned in the Italian Renaissance style by H. Van Buren Magonigle, with extensive gardens and various outbuildings, including stables and greenhouses.

After Solomon's death in 1948, IBM purchased the property in 1953, transforming it into a country club and executive training center. The property remained with IBM until it was sold to the Village of Sands Point in December 1994.”

It appears that they only used this mansion for the interiors, and then used the Duke House for the exterior shots. 
 
( A third estate and mansion was used in Episode 7, as the palatial location of Prater's country estate, reached by helicopter, where he hosts his group of serial killers in a retreat of sorts in its formal gardens. )   

There is no StreetView available for this mansion , 
but here is an image of what it looks like from the outside:

 
Here is an aerial photo of the location.  And here is a map link.