Masai became the world junior cross country champion in 2007 and set a world junior record for the 10,000 metres at the 2008 Summer Olympics, placing fourth in the final. In 2017 she was awarded the olympic bronze medal after the initial silver medallist Elvan Abeylegesse tested positive for a banned steroid in a retest of her 2007 doping sample, thus Masai became the first kenyan woman to medal in a 10,000m olympic event. She was the runner-up at the IAAF World Cross Country Championships three times consecutively from 2009 to 2011. At the 2011 World Championships in Athletics, she won her second world track medal, taking third in the 10,000 m.
