It happened on the mansion's lavish grounds in broad daylight at about 12:30 p.m.
Investigators say the gunman wore a black mask and ambushed 69-year old Gary Melius while he was getting into his car in the parking lot.
Melius survived the bullet, which apparently grazed his head.
He was rushed by a family member to nearby Syosset Hospital and was said to be alert and conscious after being transferred to North Shore University Hospital in Manhasset.
Investigators said Melius was walking to his car when he was confronted.
Ron Rosenberg, Melius's close friend and lawyer, is reeling not just at the crime, but also at the fact the his friend is already awake and talking in the hospital and expected to make a full recovery.
"One of the problems in our society is sometimes we try to look for a reason in the victim for the horrible criminal acts of the perpetrators who have no justification and whatever sick motive they may have is not a reflection of the victim," Rosenberg said.
Melius's friends just amazed at his resilience. After the shooting, Rosenberg says, he managed to stagger back to the castle and call for help.
"To realize that he was able to recover from the shot, go back into Oheka, call the police," Rosenberg said. "Somebody was watching over him. It's nothing but a miracle."
Suffolk County Detective Sgt. John O'Sullivan said that it does not appear to be an accidental shooting, but that neither the shooter nor a motive has been identified.
Detectives were scanning hours of surveillance video and visitor logs.
Oheka is the second largest private residence in America with 109,000 square feet.
It was completed in 1919 for financier Otto Kahn and then restored by Melius as a luxury hotel, catering hall, and conference center.
Melius is a prominent developer and political contributor and is well-known and well-liked.
Deborah Bryggman works at the mansion as a tour guide. "I don't know who would ever want to hurt someone so nice and generous and thoughtful. In shock," she said.
Last year, Melius was part of a controversy that resulted in the ouster of then-Nassau County Police Commissioner Thomas Dale over the arrest of a witness in an election lawsuit.
The district attorney found that Dale had instructed officers to arrest a 29-year-old Roosevelt man, who had testified in an unrelated case that he had been paid to collect signatures for a third-party candidate.
Melius, a supporter of the third-party candidate, then contacted Dale seeking to have the young man investigated. The young man was arrested on an outstanding warrant while riding a county bus on Oct. 5.
An investigation by the Nassau County district attorney found no evidence of criminal wrongdoing but raised questions about the ethics of Dale's actions, leading to his resignation.
Melius bought Oheka Castle in 1984 and soon began restoring it. He sold it a few years later, then regained ownership in 2003.
It became a hotel that has hosted celebrity weddings. Kevin Jonas, of the Jonas Brothers, married Danielle Deleasa there in 2009. Former President Bill Clinton presided over the wedding of then-Rep. Anthony Weiner and Huma Abedin, a close aide to then-Secretary of State Hillary Rodham Clinton, in 2010.
Oheka Castle also been seen as a backdrop in a number of movies, including "Citizen Kane," television shows and magazine photo shoots, and it has been used for high-profile political events and fundraisers.
The Associated Press contributed to this report.
