Morath divorced Birch and returned to Paris to pursue a career in photography. In 1953, after Morath presented her first large picture story, on the Worker Priests of Paris, to Capa, he invited her to join Magnum as a photographer. Her first assignments were stories that did not interest "the big boys." She went to London on an early assignment to photography the residents of Soho and Mayfair. Morath's portrait of Mrs. Eveleigh Nash, from that assignment, is among her best-known works. At Capa's suggestion, in 1953–54, Morath worked with Cartier-Bresson as a researcher and assistant. In 1955 she was invited to become a full member of Magnum Photos. During the late 1950s, Morath traveled widely, covering stories in Europe, the Middle East, Africa, the United States, and South America, for such publications as Holiday, Paris Match, and Vogue. In 1955 she published Guerre à la Tristesse, photographs of Spain, with Robert Delpire, followed by De la Perse à l'Iran, photographs of Iran, in 1958. Morath published more than thirty monographs during her lifetime.