David Szymin was born in 1911 in Warsaw into a family of publishers that produced works in Yiddish and Hebrew. His family moved to Russia at the outbreak of the First World War, returning to Warsaw in 1919.

After studying printing in Leipzig and chemistry and physics at the Sorbonne in the 1930s, Szymin stayed on in Paris. David Rappaport, a family friend who owned the pioneering picture agency Rap, lent him a camera. One of Szymin's first stories, about night workers, was influenced by Brassaï's Paris de Nuit (1932). Szymin - or 'Chim' - began working as a freelance photographer. From 1934, his picture stories appeared regularly in Paris-Soir and Regards. Through Maria Eisner and the new Alliance agency, Chim met Henri Cartier-Bresson and Robert Capa.

From 1936 to 1938 Chim photographed the Spanish Civil War, and after it was over he went to Mexico on an assignment with a group of Spanish Republican émigrés. On the outbreak of the Second World War he moved to New York, where he adopted the name David Seymour. Both his parents were killed by the Nazis. Seymour served in the US Army (1942-45), winning a medal for his work in intelligence.

In 1947, along with Cartier-Bresson, Capa, George Rodger, and William Vandivert, he founded Magnum Photos. The following year he was commissioned by UNICEF to photograph Europe's children in need. He went on to photograph major stories across Europe, Hollywood stars on European locations, and the emergence of the State of Israel. After Robert Capa's death he became the new president of Magnum. He held this post until 10 November 1956, when, traveling near the Suez Canal to cover a prisoner exchange, he was killed by Egyptian machine-gun fire.