Nilsson appeared at three major international championships for Sweden. In her first she ranked fourth in the 1500 metres at the 2003 European Athletics U23 Championships (originally fifth before Rasa Drazdauskaitė's disqualification). She returned to that event at the 2005 European Athletics U23 Championships, but finished a distant last in twelfth place. Her greatest international success came on her major senior debut at the 2005 European Cross Country Championships. There she had a late surge and finished with the bronze medal. This made her Sweden's second ever senior women's medallist at the competition after former champion Sara Wedlund.