Born Dawid Szymin in Warsaw, David Seymour grew up in a cosmopolitan household. His father was a leading publisher of Hebrew and Yiddish books, and he ran a bookstore that was a locus of Warsaw's Jewish intellectual life. During World War I (1914–1918) the family fled to Minsk and then Odessa before returning to Warsaw in 1919.
A passionate reader, talented pianist, and precocious linguist, Seymour passed his baccalaureate in 1929 and then studied printing technology at Leipzig's prestigious Staatliche Akademie für Graphische Künste und Buchgewerbe. He returned home after graduating in 1931, but faced with Poland's worsening economic and political climate he decided to continue his education in Paris. He enrolled at the Sorbonne in 1932 to study physics and chemistry.
Concerned about straining his family's resources, Seymour in 1933 sought work from David Rappaport, a family friend who ran a Paris-based photo agency. Though untrained in photography, Seymour was a quick learner. Soon his photographs, largely of working-class subjects, began appearing in several Parisian illustrated periodicals. He started stamping his prints "Chim," an abbreviated version of Szymin that was easier to pronounce. In 1934, he was appointed staff photographer for Regards, a leftist illustrated weekly that pioneered humanist photography in France. During these early years in Paris he also forged a lifelong friendship with Robert Capa and Henri Cartier-Bresson, two other young, politically progressive photographers.
Regards sent Chim to report on the Spanish Civil War soon after its outbreak in July 1936. His photographs of battles and especially of life behind the lines cemented his reputation as a leading photojournalist. He photographed the defeated Republicans fleeing to France and then covered the voyage of the first ship to carry Spanish émigrés to Mexico. Seymour made his way from Mexico to New York, arriving just after the beginning of World War II. Taking advantage of immigration regulations that allowed foreigners to open businesses, he teamed up with German photographer Leo Cohn to open what soon became a highly regarded photo-finishing business (called Leco) in New York. Many notable photographers who had left Europe, including André Kertész, used Leco as their darkroom.
Chim was drafted into the American army in 1942. While training in military intelligence at Camp Ritchie, Maryland, he became a naturalized U.S. citizen. Fearful of reprisals by the Nazis against his family in occupied Poland, he adopted a new, Anglo-Saxon name, David Robert Seymour. Between 1942 and 1945, Chim served in photo reconnaissance and interpretation in the U.S. Army. He worked in England, France, and eventually occupied Germany, earning several promotions and a bronze star. While in Paris to celebrate the city's liberation he found that his old apartment had been sealed by the SS, but that nothing had been removed. Soon he would learn that both his parents had been killed by the Nazis. In 1947, Seymour cofounded Magnum Photos and served as its first vice president. In 1948, he was commissioned by UNICEF to photograph the plight of Europe's children in the aftermath of World War II. The resulting images—striking and sympathetic—are among his best-known works.
In the 1950s, Seymour made Rome his home base. He became the trusted portraitist of many film stars—including Sophia Loren, Gina Lollobrigida, and Ingrid Bergman—whose images were in high demand by magazines such as Life. With a deep affinity for Mediterranean culture, he traveled frequently around Italy and Greece to pursue his own photography. He also became a dedicated documentor of the new state of Israel, with which he identified closely.
He continued to photograph regularly, even after assuming the presidency of Magnum following Capa's death in 1954. Seymour remained president until November 10, 1956, when he and French photographer Jean Roy were killed by Egyptian machine-gun fire en route to covering a prisoner exchange in the aftermath of the Suez Crisis.