She first came to international prominence at the 2004 Summer Olympics in Athens, Greece, winning a surprise silver medal behind Kelly Holmes. She repeated this feat by winning silver at the 2005 World Athletics Championships in Helsinki, Finland. At the 2007 World Athletics Championships in Osaka she cemented her place as a perennial bridesmaid with yet another impressive silver medal performance at the 2007 World Athletics Champions in Osaka, Japan. She added another Olympic medal to her impressive resume with a bronze medal at the 2008 Summer Olympicsin Beijing, China.
Hasna Benhassi is most noted for her strong finishing kick. She tends to sit back in the pack in the early stages of a race, and begin making her move only midway through the final lap. Often she successfully comes over the top of runners down the stretch, passing rivals to earn important medals. It is also her possible weakness as she often leaves herself just too much ground to make up to catch the gold medal.
In 2005, Benhassi was selected as the best sportsperson in Morocco by a survey conducted by the Moroccan Radio among 43 press institutions.