Lauder's interest in postcards led him to be acquainted with one of the owners of the Gotham Book Mart, a Manhattan bookstore, and he sought to help the Gotham re-establish its presence in the city when the owner had sold its long-time building and needed a new space. Lauder bought a building at 16 East 46th Street along with a partner, letting the building's storefront space to the Gotham. Later, the Gotham fell behind on rents, eventually resulting in Lauder and his partner to file for eviction. In a much-publicized closure of the bookstore, the New York City Marshal later auctioned the store's inventory, which was bought in a lot by Lauder and his partner to some protest from many other independent book sellers and Collectors who were present at the proceedings and hoping to purchase some of the bibliophilic treasures. Lauder has long been a major benefactor of the Whitney Museum of American Art. In 1971, he joined the museum's acquisitions board and in 1977, by then President of his family's Business, he became a Whitney trustee. He became President in 1990 and has been chairman since 1994. He has donated both money and many works of art to the Whitney, and is the museum's most prolific fundraiser. His 2008 donation to it of $131 million is the largest in the museum's history. Through the Leonard and Evelyn Lauder Fund, he and his wife have also sponsored several exhibitions at the Whitney. The fifth-floor permanent collection galleries are named for the couple. In 1998, he told a reporter for The New York Times that his "dream job" was to be the Whitney Museum's Director. Most recently, Lauder gave $131 million for the Whitney's endowment.
He is the chairman emeritus of The Estée Lauder Companies Inc. He was chief executive until 1999. Today, Estée Lauder operates several brands in the cosmetic industry, including Estée Lauder, Clinique, MAC Cosmetics, Aveda, Bobbi Brown, Smashbox and La Mer.
Lauder gained notoriety in 2001 for creating the Lipstick index, a since discredited economic indicator.
In autumn, 2012, the Museum of Fine Arts, Boston opened an exhibition of 700 of his postcards, a tiny part of the promised gift he has made to the museum of 120,000 postcards: The Postcard Age: Selections from the Leonard A. Lauder Collection. In an interview in The New Yorker, Lauder explained how postcards turned him into a collector, and how these "mini-masterpieces" remained his lifelong pursuit to the point where his late wife, Evelyn, called the collection his "mistress".
A long-time supporter of the Metropolitan Museum of Art in New York City, Lauder led the creation of a research center for Modern art at the museum, which he helped support through a $22 million endowment made alongside museum trustees and other benefactors. In April 2013, he promised his collection of 81 pieces of Cubist art, consisting of 34 pieces by Pablo Picasso, 17 by Georges Braque, 15 by Fernand Léger, and 15 by Juan Gris to the museum; together, they are valued at more than one billion dollars. It has been described by william Acquavella, of Acquavella Galleries, as "without doubt the most important collection any private person has put together in many, many years,”
On January 1, 2015, he married Photographer Judy Ellis Glickman.