Young died on October 6 in Los Angeles, his daughter told the New York Times. No reason was given.
Young's role as Paulie Pennino in the six Rocky films remains his most famous achievement.
When Paulie first appeared in the original 1976 film Rocky, he was an angry, foul-mouthed meatpacker who was abusive to his sister Adrian, with whom he shared a small apartment in Philadelphia.
She scolds the shy and meek Adrian for initially refusing to go on a Thanksgiving night date with his friend and co-worker Rocky Balboa, and destroys the turkey in the oven.
The film became a phenomenon, topping the box office this year and making a star of its lead actor and writer Stallone, who posted a tribute to Young on Instagram Wednesday night.
Rocky was nominated for 10 Oscars, including best supporting actor, for Young. It won three awards, including best picture.
Young and leading actor Burgess Meredith, who was also nominated, lost to Jason Robards in All the President's Men.
As the films progressed, Young's Paulie softened and became their comic relief, as in the sequels.
In 1985's Rocky IV, she reprograms the robot Rocky gave her into a sexy-voiced maid who cares about him.