Comăneci was discovered by Bela Karolyi, later the Romanian gymnastics coach, when she was six years old. She first competed in the national junior championships in 1969, placing 13th, and she won the competition in 1970. Her first international competition was in 1972 in a pre-Olympic junior meet for the communist-bloc countries in which she won three gold medals, and in 1973 and 1974 she was all-around junior champion. In her first international competition as a senior in 1975, she bested the Russian Lyudmila Turishcheva (Turishcheva, Lyudmila Ivanovna), the five-time European champion, winning four gold medals and one silver. She won the American Cup in New York City in 1976, becoming the first woman to perform a backward double salto as a dismount from the uneven parallel bars.

At the 1976 Olympic Games in Montreal, Comăneci received seven perfect scores and won the gold medals for the balance beam, the uneven bars, and the all-around individual competition. She won a silver medal as a member of her team and a bronze medal for the floor exercises. After the 1976 Games, she was named a Hero of Socialist Labour by her country. The song used to accompany her floor exercises was retitled "Nadia's Theme (The Young and the Restless)" and became an international hit, earning a Grammy Award in 1977. She finished a disappointing fourth in the world championships in 1978, however, and was out of competition during most of 1979 with an infected hand. At the 1980 Olympic Games in Moscow, she won gold medals for the beam and the floor exercises (tying for first in the latter event with Nelli Kim of the U.S.S.R.). She won a silver medal as a member of her team and tied with Maxi Gnauck of East Germany for second place in the all-around individual competition. She retired from competition in 1984.

Comăneci defected to the United States in 1989; she became a U.S. citizen in 2001. In 1996 she married American gymnast Bart Conner, with whom she works to promote gymnastics. She published an autobiography, Nadia (1981), and a book on mentoring, Letters to a Young Gymnast (2003).