István Kausz began his modern pentathlon career representing Vasas in 1949. From 1952 he competed for Budapest Haladás and won the winter pentathlon team championship three years in succession, 1952-54. In 1954, he switched to épée fencing, as an OSC competitor. Between 1957-65, he reached the top as a member of the Hungarian fencing team. He was a world champion at both individual (1962) and team (1959), and was also a double silver- and single bronze medalist in the team event. Kausz was also a Universiade winner in 1959.
Kausz was also a two-time Olympian, first in Rome, where he finished fourth in the team competition, but his greatest achievement was at Tokyo in 1964, where he won a gold medal in the team event.
He obtained his medical degree during his active sporting career, and became a doctor at the Central Sports School and then for the Hungarian swimming team. As a member of the medical team, he represented the Hungarian aquatic team at several Summer Olympics, from München in 1972 to London 2012.