Born in Ho Chi Minh City, Thanh Hang won a national title over 1500 metres in 2004 and began to represent her country internationally the following year, beginning with a sixth-place finish at the 2005 Asian Athletics Championships. She won the 1500 m gold medal at the 2005 Southeast Asian Games, breaking the Games record and Vietnamese national record, and she also claimed the bronze medal in the women's 800 metres. Her next major competition was the 2006 Asian Games and she was knocked out in the qualifying rounds of the 800 m, but took fourth in her 1500 m speciality, improving her national mark to 4:17.66 minutes in the process.