The mother of policeman Peter Figoski held her hands over her mouth and cried Wednesday as Kevin Santos was pronounced guilty.
About two dozen officers applauded the prosecutors outside the Brooklyn courtroom.
Figoski was shot as officers interrupted the robbery of a drug dealer.
The resident of West Babylon, Long Island, was promoted to detective after his death.
The two other defendants, Nelson Morales and shooter Lamont Pride, were convicted of murder at separate trials.
Santos' lawyer said he will appeal.