After immigrating to Indonesia with his family when he was a child, Widjaja became a member of the PCC, traded copra in the mid-1950s, moved into the palm oil industry soon after, started a paper factory in the 1970s, and then entered financial services in the 1980s.
At the time of his death, Sinar Mas had interests in paper, real estate, financial services, agribusiness, and telecom with holdings primarily in Indonesia, Singapore, Malaysia, and China, and Widjaja was listed by Forbes as the third-richest person in Indonesia with an estimated net worth of US$8.6 billion.
[10] In his early career, Widjaja did various business, including trading cooking oil and agricultural products, coffee shop, pig rearing, bakery, and grave construction.
When Indonesia's war for independence against the Dutch crushed his commodity-trading business in 1949, he sold family jewelry to repay creditors and traded in his car for a bicycle.
[4][14] In 1972, together with Taiwanese investors, Widjaja acquired caustic soda producer Tjiwi Kimia, which he transformed into the Sinar Mas Group's first pulp and paper manufacturer.
[16][17] Also in 1982, Widjaja acquired Bank Internasional Indonesia (BII)[18] and founded PT Internas Artha Leasing Company.
[2][13] After the 1997 Asian financial crisis and a dip in the international wood pulp price in 2000, it was revealed that the company had a global US$13.9 billion debt.
The eldest daughter, Sukmawati Widjaja (Oei Siu Hoa), serves as Sinar Mas vice chairperson since 1988.
[33] Indra Widjaja's son, Fuganto Widjaja, heads coal mining and trading company Golden Energy and Resources, Sinar Mas' Singapore-based subsidiary, and is considered as the group's new face, proclaimed to shift its focus to energy and infrastructure, telecoms, healthcare, and education.
[31] A son from another marriage, Oei Hong Leong, is ranked by Forbes in 2018 as the 22nd richest person in Singapore, with a net worth of US$1.5 billion.