Hariri was born on 1 November 1944 to a modest Sunni Muslim family, along with two siblings (brother, Shafic and sister Bahia) in the Lebanese port city of Sidon. He attended elementary and secondary school in Sidon, and graduated in business administration at Beirut's Arab University. In 1965, Hariri left his home and went to Saudi Arabia. There, he earned from teaching for a short period of time, later shifting to the construction industry. In 1978, he gained Saudi Arabian citizenship, in addition to his Lebanese citizenship.
In 1969, Hariri established Ciconest, a small subcontracting firm, which went out of business soon. He then went in business with the French construction firm Oger for the construction of a hotel in Ta’if, Saudi Arabia, the timely construction of which brought him in good graces with King Khaled. Hariri took over Oger, forming Saudi Oger, which became the main construction firm used by the Saudi Royal family for all of their important developments. As a result, only a few years after his first contract with King Khaled, Hariri had become a multi-billionaire.
Having accumulated his wealth, Hariri started a number of philanthropic projects, including the building of educational facilities in Lebanon. His first initiative in Lebanon was the Islamic Association for Culture and Education which was founded in 1979. The association was later renamed the Hariri Foundation. Hariri became progressively more embroiled in politics. His appeals to the U.N. and services as an emissary to the Saudi Royal family won him international recognition on the political stage for his humanitarian efforts but also slowly caught him in the web of Lebanese politics.
In 1982, he donated $12 million to Lebanese victims of the 1978 South Lebanon conflict and helped clean up Beirut's streets with his company's money. After the conflict, he acted as an envoy of the Saudi royal family to the country. He laid the groundwork that led to the 1989 Taif Accord, which Saudi Arabia organized to bring the warring factions together. Taif put an end to the civil war and paved the way for Hariri to become prime minister.

Source: Wikepedia