Born on June 24, 1923, in St.-Genis-Laval, near Lyon, he was the fifth and, by his account, the most shy of seven children from a bourgeois family that expected him to take up a respectable vocation. It was his father, an enthusiastic traveler and amateur photographer, who led him astray by giving him a vest-pocket Kodak when Marc was a teenager.

His first photographs were of the Paris Exposition in 1937. After World War II, in which he fought around Vercors as a member of the Resistance, Mr. Riboud studied mechanical engineering at the École Centrale in Lyon. He took a factory job in the nearby town of Villeurbanne after graduating in 1948.

Not until he found himself taking pictures of a cultural festival in Lyon during a weeklong vacation in 1951 did he at last decide to commit to the unstable life of a freelance photojournalist. He moved to Paris in 1952.

There he met Cartier-Bresson, who became his mentor. Already a celebrity in his field, this “salutary tyrant,” as Mr. Riboud called him, dictated “which books to read, what political ideas I should have, which museums and galleries to visit.”

“He taught me about life and about the art of photography,” Mr. Riboud said.

Among the lessons imparted was that “good photography” is dependent on “good geometry.” The Eiffel Tower photograph from 1953, the first that Mr. Riboud published, proves how well the pupil absorbed the lesson. In a radio interview more than 50 years later, he still recalled the English-language caption given to the image by the Life copy writers: “Blithe-ful on the Eiffel.”

In 1953, Cartier-Bresson nominated his protégé to join Magnum, the photo collective he had helped found. Until 1979, when he left to go out on his own, Mr. Riboud traveled and photographed for the agency constantly.