Nina Kuscsik began jogging in 1967 using Bill Bowerman's JOGGING book as a guide. She began running competitively in 1969 when she entered the Boston Marathon as an unofficial entrant, finishing in 3:46. She completed 80 marathons with a best time of 2:50:22. Kuscsik set an American record for 50 miles in 1977 in 6:35:53. She represented the United States in international competition in Germany and Puerto Rico.

Kuscsik was the first woman to run in the New York City Marathon, the first female winner of the Boston Marathon and a two time winner of the New York City Marathon. Her winning time for the 1972 New York City Marathon was on the slow side since the women had a sit-down strike for ten minutes after the gun went off to protest women's inequality in marathon running.

Kuscsik introduced changes in the athletic rules to allow women to run the marathon distance in the United States and to hold United States Championships. She also prepared and introduced resolution adopted by the USA and the International Amateur Athletic Federation (IAAF) to place the women's marathon in the Olympics. She presented beginning, marathon and women's running clinics to athletic and professional organizations throughout the United States. Kuscsik wrote training articles for Runner's World, Women's and Fitness Magazines.

Kuscsik was a commentator for running events and training commentary with 1010 WINS and CBS Radio, New York.