Pam Hearne heard "a loud crash" and later was stunned to see a foot clad in an Adidas sneaker and a sock in her yard, said Officer Thomas Blanchard. The leg, with hip and spine attached, dented the shingled roof of her garage before bouncing into the lawn.
Police suspect the remains are from a stowaway who may have been crushed as the South African Airways jet lowered its landing gear on its approach to Kennedy Airport.
Federal Aviation Administration spokesman Jim Peters said the pilot of flight 203 would have probably started lowering the plane's landing gear in the sky over the home in Floral Park, about 5 miles from the airport.
Peters said a Customs agent that met the flight at the airport found another leg hanging from the wheel well.
The airline said in a statement that the flight landed with "no impact" on the passengers and crew and it was working closely with authorities to investigate how someone may have stowed away.
The flight originated in Johannesburg, and made one stop in Senegal. Authorities had not identified the remains, which were hauled away from Hearne's yard in a plastic bag.
Hearne, a special education teacher, said that when she first saw the leg in the grass, "it didn't look real."
"But I am very glad that I live where I do," she said, "so I don't have to run for my life like this man probably was doing."
There have been cases of stowaways being crushed by the mechanism in aircraft wheel wells and perishing from the extreme cold at high altitude.