Mårtensson represented Sweden at the 1972 Summer Olympics in Munich, West Germany, at the remarkably young age of 11 years old. Despite her youth, she participated in three events during the Games:
- 100 meter butterfly: She competed in the heats, finishing 7th in her heat and not advancing to the semifinals.
- 4 × 100 meter freestyle relay: As part of the Swedish team, she helped secure a place in the final, where the team finished in 7th position.
- 4 × 100 meter medley relay: She also swam in the medley relay, where the Swedish quartet reached the final and placed 5th.
Mårtensson's Olympic appearance at such a young age is a notable aspect of her swimming career.