Masai was born in Chepkoya village, Mount Elgon District. She is the third born from a family of four. She competed during her high school days while at Kibuk Secondary School. She graduated in 1988. She joined Kenya Prisons Service in 1990, known for recruiting many talented athletes. Masai did not, however, achieve anything until 1999, when she became the national cross-country champion, at the age of 32. Consequently, she started training more seriously. She won the Lotto Cross Cup Brussels in 2001 and went on to take bronze in the short race at the World Cross Country Championships that year. She improved upon this the following year, taking the gold at the 2002 IAAF World Cross Country Championships.