Stock served in the United States Army from 1947 to 1951. In 1951 he became an associate member of the photography agency Magnum. He became a full partner-member in 1954. In 1968, Stock left Magnum to start his own film company, Visual Objectives Inc., and made several documentaries, but he returned to the agency a year later, as vice president for new media and film. In the mid-1970s, he traveled to Japan and the Far East, and also produced numerous features series, such as photographs of contrasting regions, like Hawaii and Alaska. In the 1970s and 1980s he focused on color photography of nature and landscape, and returned to his urban roots in the 1990s focusing on architecture and modernism.
In 2006 Stock married writer Susan Richards. They lived in Woodstock, New York with their four dogs.