At age 15 Kroc lied about his age in order to join the Red Cross ambulance service on the front lines of World War I. He was sent to Connecticut for training, where he met fellow trainee Walt Disney (Disney, Walt), but the war ended before Kroc could be sent into service overseas. Kroc then returned to Chicago and held various jobs throughout the 1920s and '30s, including jazz pianist, real-estate salesman, and paper-cup salesman for Lily-Tulip Cup Co. In the early 1940s he became the exclusive distributor for the “multimixer,” a blender that could simultaneously mix five milk shakes. In 1954 he visited a restaurant in San Bernardino, California, that used eight of his mixers. The restaurant was owned by two brothers, Maurice and Richard McDonald, who used an assembly-line format to prepare and sell a large volume of hamburgers, french fries, and milk shakes. Impressed by what he saw, Kroc decided to set up a chain of drive-in restaurants based on the McDonald brothers' format, and he agreed to pay the brothers a franchise fee based on gross receipts from the new restaurants.

By the time Kroc bought out the McDonald brothers in 1961 for a mere $2.7 million, he had established 228 restaurants and sales had reached $37 million. By the end of 1963 the company had sold more than one billion hamburgers, an achievement proudly advertised beneath its restaurants' trademark “golden arches.” At the time of Kroc's death in 1984, there were some 7,500 McDonald's outlets worldwide, and three-fourths were run by franchise holders.

Kroc served as president of McDonald's from 1955 to 1968, as chairman of the board from 1968 to 1977, and as senior chairman from 1977 until his death. During his lifetime Kroc was an active supporter of numerous charitable organizations. From 1974 he was the owner of the San Diego Padres Major League Baseball team. The team was sold by his widow, Joan Kroc, in 1990.