Kirszenstein was born in Russia, although her parents were Polish, and the family returned to Poland when she was still a child. She was 18 years old when she competed at the 1964 Olympic Games in Tokyo, earning a gold medal in the 4 × 100-metre relay and silver medals in the 200-metre run and the long jump. In 1967 she married Janusz Szewińska, a sports photographer. At the 1968 Games in Mexico City, she won the 200-metre run in world record time (22.5 seconds) and earned a bronze medal in the 100-metre event.

The birth of her son Andrej in 1970 and a severe ankle injury kept Kirszenstein-Szewińska from training for a year, but she was able to recover in time to attend the 1972 Olympics in Munich. There she took the bronze medal in the 200-metre run. In 1973 her husband became her coach, and she decided to compete in a new event—the 400-metre sprint. The following year in Warsaw she became the first woman to run 400 metres in less than 50 seconds. At the 1976 Games in Montreal, she won a gold medal in the 400 metres in world record time (49.28 seconds). Her Olympic career was ended by a muscle strain at the 1980 Games in Moscow.

Named Poland's Athlete of 1965, Kirszenstein-Szewińska also earned Woman Athlete of the Year honours from the Soviet news agency Tass and many other organizations during the 1960s and '70s.