65-year-old Rawat was administered the oath of office by Governor Aziz Qureshi at Raj Bhawan lawns along with a 11-member Cabinet all of whom were ministers in the Bahuguna government.
Rawat's name was approved by Congress president Sonia Gandhi, senior party leader Janardhan Dwivedi told reporters here after a five-hour-long meeting of the Congress Legislature Party. Rawat was Union Cabinet Minister for Water Resources.
His appointment comes at a time when Congress is trying to salvage its image in the hill state.
Bahuguna, who resigned on Friday, has drawn intense criticism for the way he handled last year's devastating floods and relief and rescue work.
In the 70-member assembly, Congress has 33 members and has the support of seven-member PDF including three BSP, one UKD and three Independent MLAs. BJP has 30 MLAs.
The PDF, which has four ministers in the outgoing Bahuguna ministry, has pressed for additional ministerial berths in the new government.
Rawat pipped two other front runners for the top post -senior party leader Indira Hridayesh and member of Bahuguna's cabinet Pritam Singh.
Spelling out his priorities, Rawat said it would be definitely to pull the state out of the fallout of the 2013 catastrophic flashfloods.
Besides Dwivedi, senior Congress leaders Ambika Soni and Ghulam Nabi Azad attended the CLP meet as central observers.
Having the reputation of a hard-working leader, Rawat, MP from Haridwar, was made a Minister of State for Labour and Employment in 2009 and then MoS for Agriculture and Food Processing Industries. In July 2011, he was made MoS for Parliamentary Affairs.
He was elevated to the rank of a Cabinet Minister in 2012 in an apparent bid to placate him after Bahuguna was made the CM.
Rawat has earlier served as a member of the Rajya Sabha from 2002-08. He won his first Lok Sabha election in 1980 from Almora by defeating BJP leader Murli Manohar Joshi. He won the Lok Sabha polls in 1984 and 1989 as well.
The eleven members of Rawat's Cabinet are -- Harak Singh Rawat, Indira Hridayesh, Yashpal Arya, Surendra Singh Negi, Pritam Singh, Amrita Rawat, Dinesh Aggarwal, Mantri Prasad Naithani, Pritam Singh Panwar, Harish Durgapal and Surendra Rakesh.
There are no fresh faces among Rawat's ministerial colleagues with all those sworn in tonight also being part of Vijay Bahuguna's Cabinet.
Rawat had missed the chance of occupying the top office twice in the past. He had lost the race for chief ministership once each to N D Tiwari in 2002 and to Vijay Bahuguna in 2012.
A one-line resolution passed unanimously by legislators at the CLP authorised Sonia Gandhi to chose its leader saying her choice will be acceptable to all.
Explaining the delay in building a consensus over Rawat's name, he said it took some time to get the AICC chief's approval as she was on a tour to Karnataka.
The swearing-in ceremony, originally scheduled to take place at 3:45 PM, was delayed with the CLP meeting stretching to a little over five hours and concluding finally at 5:15 PM.
Though some reports attributed the delay to insistence of the seven-member PDF on ministerial berths, Dwivedi said the delay was caused as it took time to hear from Gandhi who was on a tour of Karnataka.
The PDF comprises three each BSP and Independent MLAs and one UKD member.
Some other names, including that of senior party leader Indira Hridayesh and member of Bahuguna's Cabinet Pritam Singh, also did the rounds but Rawat had an edge over them in terms of stature.
Soon after he was chosen the CLP leader, Rawat thanked Sonia Gandhi and other senior party leaders, including Indira Hridayesh and Bahuguna, for supporting him.
Earlier, Rawat arrived to a rousing reception by his supporters at the Dehradun railway station in the wee hours.
He headed straight to the residence of party MP Satpal Maharaj to take his blessings and then went to a hotel where a large number of his supporters gathered to garland him amid beating of drums and presentation of Kumaoni folk dances.