The shooting involved an attempted robbery at a Tourneau luxury watch store, sources told ABC station WABC-TV.
When the gunman fired a round, the victim, a 67-year-old mall employee, was hit by a stray shot, a source told ABC News.
The victim was hospitalized in serious but stable condition, a source told ABC News.
Deputy Inspector Kenneth Lehr of the NYPD fugitive enforcement division, who happened to be shopping at the mall, immediately jumped in and apprehended the suspect, a source said. A store security guard, also a retired detective, aided in the apprehension, said a source.
It appears the shooting took place at a Rolex section of the Tourneau store, a source briefed on the investigation told ABC News.
The mall never closed and was never was put on lockdown, the source said, but nearby schools were placed in a "lockout," according to a tweet from the Garden City school district.
Roosevelt Island Mall has more than 270 stores, including five department stores, according to its website.
"The mall is cooperating fully with the Nassau County Police Department in its ongoing investigation," Simon Malls said in a statement. "The safety of our tenants and shoppers remains a priority at all times."