"I own responsibility for my party's defeat," he told reporters here.
He was categorical in accepting the mandate by saying: "It's the people's decision to make one victorious. This is their right."
Six-time Chief Minister, Mr Singh, who won his Arki seat with a margin of 6,051 votes, blamed the party's central leadership for lack of campaigning.
"Whatever are the shortcomings, I did my best during campaigning alone in the state and that too, within my resources," he added.
He refused to put the blame on Congress President Rahul Gandhi. "The elections were conducted under my leadership and I accept our defeat."
Congress cabinet Ministers Sudhir Sharma, Thakur Singh and Prakash Chaudhary faced defeat from the Bharatiya Janata Party.
The BJP was set to wrest power in Himachal Pradesh with its candidates on a victory lap in 36 of the 68 seats in the Assembly elections. The party won 21 seats as counting of ballots progressed.
However, BJP's state chief Satpal Singh Satti faced a humiliating defeat, while its Chief Ministerial candidate Prem Kumar Dhumal was trailing with a huge margin.
The ruling Congress managed to win only 10 seats.