The group wants to turn the ship into a historic attraction, permanently set on the waterfront of a major city.
The Lenfest donation has enabled the conservancy to enter into an exclusive purchase-option agreement with Norwegian, officials said.
"This is a game-changer in our work to save this irreplaceable American icon," said conservancy board President Susan Gibbs, granddaughter of the ship's designer, William Francis Gibbs. "Mr. Lenfest's vision and generosity give us a fighting chance to preserve the SS United States for generations to come."
She credited the ship's owners, who "turned down higher offers to partner with us in this patriotic effort."
The SS United States still holds the trans-Atlantic speed record, set 58 years ago on her maiden voyage from New York to England. The ship transported four men who were or who became U.S. presidents, along with countless heads of state, military, and business leaders. It also brought immigrants to these shores.
The ship has had numerous owners since being removed from service in 1969. Earlier this year, Norwegian began accepting bids from scrap firms, bringing new urgency to preservation advocates.
Lenfest has been interested in the project. Last year, he pledged $300,000 toward the purchase of the ship.
"Having established a relationship with the conservancy in 2009, Gerry Lenfest literally swept in at the 11th hour to save this national treasure," said Dan McSweeney, executive director of the organization. "Mr. Lenfest understands this ship is a symbol of American preeminence in the 20th century. She can also become a part of America's future promise, potentially creating thousands of jobs during and after refurbishment."
