Mary Decker was born on August 4, 1958, in Bunnvale, New Jersey, although her family moved to Southern California ten years later. It was in California that Decker first got interested in running. At age eleven, a year after taking up running, Decker won her first local racing competition. She immersed herself in running, competing in several other local and regional events. She also joined a running club and ran for her school teams. Decker showed an amazing determination and pushed herself to the limits of her endurance. During one week when she was twelve, Decker completed a marathon and four middle- and long-distance races, ending the week with a trip to the hospital to get an appendectomy.
By the time she was in her early teens, Decker had won enough competitions and posted low-enough times that she was recognized as a world-class runner. Despite this fact, Decker, nicknamed Little Mary (because she was under five feet tall and weighed only eighty pounds) was ineligible to attend the 1972 Olympic trials, since she was too young. This did not stop Decker from competing with Olympians, however. Decker won the 800-meter event in her first international competition later that year—beating the silver-medal winner from the 1972 Olympics in the process.
By 1973, Decker was ranked first in the United States and fourth in the world in the 800-meter race. The same year, she scored her first record, by running an indoor mile in 4:40.1. A year later, thanks in part to a tremendous
growth spurt, Decker developed a case of compartment syndrome, a muscle condition. In 1978, Decker had leg surgery to correct this problem, and the surgery kept her out of competition temporarily.