NEW YORK—Ileana Sonnabend, one of the "most formidable contemporary art dealers of her time," died Sunday in her sleep after a long illness, the New York Times reports. She was 92.

Sonnabend, born Ileana Schapira in Bucharest, Romania, started her career in the 1960s, and she became known for introducing postwar American art to Europe, keeping Americans informed on developments abroad, and amassing a huge art collection that mostly remained in storage. She ran two galleries—Sonnabend Gallery in New York and Galerie Ileana Sonnabend in Paris—which sold works from an illustrious list of artists including Jasper Johns, Andy Warhol, and Jeff Koons and mounted much-talked-about exhibitions for more than 40 years.

Her first husband, Leo Castelli, also was a prominent art dealer. When Sonnabend married him at the age of 18, she asked for and received a Matisse instead of an engagement ring.