Banharn was born on Aug 19, 1932 in Suphan Buri to a Chinese merchant family. He dropped out of high school when the Imperial Japanese Army invaded Thailand during World War II. He went to work with his older brother, then founded his own building company.
The construction company became highly successful in the 1960s from work on government infrastructure projects, including military facilities, and Banharn became very wealthy. He also came to learn that while obtaining government construction contracts could be lucrative, being in government and in charge of awarding contracts was also very attractive.