Jurors found Marc Ferrara, 43, guilty of aggravated manslaughter Thursday in the 1982 bludgeoning death 17-year-old Rosie Lorincz.
Prosecutors had sought a murder conviction, but jurors were allowed to consider the lesser charge. Because of a 1993 state Supreme Court ruling, the panel was not told that their manslaughter conviction would mean no punishment.
The law setting a statute of limitations on manslaughter charges, barring prosecution after five years, has since been changed, but it was not made retroactive to apply to older cases.
A body surfaced in the Hudson River nine days after the killing, but it was not identified at the time and was buried in a potter's field in New York City.
Then in 2002, Ferrara's sister, Phyllis, told police her brother forced her to help get rid of Lorincz's body. The body found two decades earlier was exhumed and identified as Lorincz through DNA testing.