The tribal leader was administered the oath of office and secrecy by Governor Syed Ahmed at the Birsa Mandap situated in the premises of Raj Bhavan.
Rajendra Prasad Singh and Annapurna Devi, Legislature Party leaders of the Congress and the RJD respectively, were also sworn in as ministers.
Hemant Soren, who heads the ninth ministry in less than 13 years, became the fifth tribal CM after Babulal Marandi (once), Arjun Munda (thrice), Shibu Soren (thrice) and Madhu Koda (once) since the state was created on November 15, 2000.
The JMM-LP leader had staked claim on July 9 for formation of a government in alliance with Congress, Rashtriya Janata Dal, smaller parties and independent MLAs.
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Soren, who gave the Governor a list of 43 MLAs supporting him in the 82-member Assembly, is expected to expand his ministry after taking the floor test.
The alternative coalition comes nearly six months after the imposition of President's rule, which was to end on July 18.
The JMM had extended support to the BJP on September 11, 2010, ending the President's rule then. The central rule was imposed after the BJP had pulled out from the Shibu-Soren led alliance on May 24, 2010, as JMM chief had voted along with the UPA in a cut motion in Lok Sabha that year.
This is the third government in the same house, which was constituted after the results threw a fractured mandate in the November/December 2009 Assembly elections.