Marina Stepanova started to compete internationally in 1978 at the European Championships in Prague where she finished sixth in her semifinal heat. In this year she had a PB of 56.19 seconds in the 400m hurdles. In 1979 she broke the world record for the first time (54.78 seconds) beating Tatyana Zelentsova the current world record holder at the time, she would eventually break the record two more times. In 1981 she retired to give birth to a daughter. She returned in 1983 and quickly regained her form. In 1984 she tied her PB and later brought it down to 54.34 seconds then 53.67 seconds. At the European Championships in 1986 won the gold medal in a world record time of 53.32 seconds. 3 weeks later in Tashkent, Marina became the first woman in history to run under 53 seconds in a time of 52.94 seconds in the 400m hurdles. She was 36. After the 1987 season she retired at the age of 37.
Marina Stepanova was never able to compete at the Olympics. The 400m hurdles was not introduced for women until 1984 in Los Angeles and then she could not compete because of the Soviet boycott and by the 1988 season she had retired so she could not compete in Seoul either.