76-year-old Singh, who nearly risked his government on the nuclear deal, made it through this election unscathed largely on the strength of his image as a man of integrity.
Singh is widely regarded as one of the cleanest persons in public life, but has never won any election and hence was nominated by his party to the upper house of Parliament.
The only other leader who returned to the top job after a five- year-term was Jawaharlal Nehru.
Congress' victory came as great relief for Singh, who the Left parties accused of compromising India's sovereignty to the US over the Indo-US nuclear deal, while the opposition had criticised him as acting as a puppet prime minister.
"Its all destiny...if you are destined to serve the country, you will," Singh's wife Gursharan Kaur had said in a interview last month about her husband landing the top job.
The civil nuclear deal aimed at securing nuclear fuel to run the country's atomic power plants at full capacity is Singh's key achievement as Prime Minister and ended India's decades of isolation by the international nuclear community.
Singh's greatest contribution, however, is the economic reforms process that he set in motion in the early 1990s.
A D.Phil in economics from Oxford University and author of 'India's Export Trends and Prospects for Self-Sustained Growth' - an early critique of India's inward-oriented trade policy - Singh opened the Indian economy, cut taxes and red tape to set the country on the path of high economic growth.
As a result of debt build-up in the early 90s, India was on the verge of default in respect of external payment liabilities, its capital market went into a tailspin, and its sovereign ratings slashed.
It was Singh, who was roped in by P V Narasimha Rao as Finance Minister, who initiated macro-economic reforms that included opening the market to foreign investment and bringing in precious foreign exchange.
There has been no looking back for Singh since then and the high point came in 2004 when Congress President Sonia Gandhi did the 'renunciation act' and nominated the mild- mannered leader as the Prime Minister.
"It was an unbelievable moment-since he was not contesting, he was not trying, he was out of the race...it came out of the blue," Kaur said of the day he was named PM.
The victory is also sweet revenge on his opponents, who dubbed him as a weak prime minister.
In fact, Singh has emerged stronger with more room to go ahead with Congress' policies, including on the economic front, especially at a time when the global economy is plunged in a crisis.
The Prime Minister was "extremely worried" about a fractured mandate as that would impact the stability of the government given the economic crisis and security situation in the neighbourhood, Minister of State in the PMO Prithviraj Chavan had said after the results gave the UPA a clear edge.
The feat is all the more creditable because he has led a coalition government, the first experience of sharing power by the Congress.
Known as "Doctorsaab" in the Congress circle, Singh has been in the higher echelons of the party since 1991 when he was persuaded by Rao to become his finance minister.
Before becoming the Prime Minister, Singh was leader of the Opposition in Rajya Sabha. Singh has been a member of Rajya Sabha from Assam and has been a member of the AICC from Assam as well as Punjab.
A devout Sikh, Singh and Gursharan Kaur have three daughters.
The Prime Minister, who underwent cardiac by-pass surgery a few months ago, did not contest the Lok Sabha elections apparently to avoid the rough and tumble after the major operation.
Though Congress leaders from Punjab had requested him to enter the fray from any constituency in the state, especially Amritsar, Singh had decided to stay away.
