Singh, the economist credited with playing a key role in ushering in economic reforms in the 1990s, demits office after 10 years, leaving a mixed legacy of achievements and failures.
Scams that surfaced during UPA 2 may have undone the good work of the 81-year-old leader, who had achieved the distinction of serving two tenures as the Prime Minister, the longest after the first PM Jawaharlal Nehru's 17 years in office.
After Friday's counting of votes in the Lok Sabha elections in which the UPA is projected by exit polls to suffer a massive and comprehensive defeat, Singh will tender his resignation on Saturday.
A celebrated economist, he entered politics at the height of the 1991 economic crisis when late prime minister PV Narasimha Rao inducted him into the government as the finance minister.
Together they lifted the economy out of the balance of payments crisis and then paved the way for the economic reforms on which no successive government has looked back.
A technocrat who had occupied various positions including as Reserve Bank governor and secretary general of the South-South Commission, he had earned a name for probity and integrity that made him the automatic choice for Congress president Sonia Gandhi when she decided to renounce the post of the Prime Minister.
Taking over as Prime Minister from the NDA government in the aftermath of the 2002 post-Godhra riots and a surcharged communal atmosphere, Singh's administration brought in a sense of balance in the situation.
As he ends his decade-long tenure as head of two successive UPA governments, his stock as a middle class hero stands severely diminished due to a floundering economy, shrinking opportunities and the acts of omission and commission of colleagues in the government and party.
Yet, Singh stands out in eyes of those who worked with him and knew him closely as a visionary and statesman who pushed through policy moves that he felt were critical for the country in the long run, had a strategic conception of a rising India's place in the world while also keeping the shaky UPA boat afloat till the end, swallowing his own pride many a time in the process.
He exhibited rare daring and passion at least twice in his public life -- once when as finance minister he sculpted India's economic reforms in 1991 in the face of a balance of payments crisis, and then, at the fag end of UPA 1, when as the Prime Minister he determinedly pushed through the landmark India-US nuclear deal.
A key proponent of inclusive growth, whose economic ideology of combining development with social equity was shaped by his teachers at Cambridge, Singh - who also studied at Oxford and taught at the Delhi School of Economics - piloted the key welfare initiative in the shape of the rural employment guarantee scheme.
The scheme that guaranteed a member of every rural poor household 100 days' employment in local infrastructure and development projects was a hit and helped the prime minister win a second term with 200-plus seats.
But, according to his former media advisor Sanjaya Baru, the credit for the scheme was hijacked by the Congress party, which sought to showcase it as the achievement of Congress vice-president Rahul Gandhi.
Baru says an attempt by him to play it up as the Prime Minister's birthday gift to the nation on Sep 26, 2007, fetched him an admonition from Singh.
"Let them take all the credit. I don't need it. I am only doing my work," he said, Baru recounts in his book The Accidental Prime Minister: The Making and Unmaking of Manmohan Singh.
That line reveals Singh's keenness to always remain in the shadows, behind party president Sonia Gandhi, never striving to project himself more than her or the Gandhi family. After all, the family appointed him as the Prime Minister in the first place over many other aspirants.
"Dr Singh had a powerful story to tell about his achievements as Prime Minister, but he invariably shied away from telling it...'I want my work to speak for me'," Baru quotes him as saying in his book.
Though criticised at home and described by the opposition as "weak, indecisive and heading a government that was most corrupt", Singh was a different person during his diplomatic trips abroad - confident and in charge, not hobbled by the pulls and pressures of party and coalition politics.
BK Chaturvedi, who was cabinet secretary from 2004 to June 2007 and worked closely with Singh, says that the "enormous respect which the global community, including China, USA, Russia and other major powers gave to India was due to his personality and enormous depth of knowledge of economic issues".
"His quiet handling of the 2006 tsunami and other major disasters, his humility and enormous courtesy to his colleagues, coupled with depth of knowledge and his continuous emphasis on introducing reforms. He is a Prime Minister with great vision for India," Chaturvedi told IANS.
Outlining his legacy, Chaturvedi said: "The Prime Minister's legacy to the government is, first, handling the nuclear apartheid of India and making it part of the international community, increasing per capita incomes three times during the last decade, sharp reduction in poverty, rapid increase in rural consumption, enhancing citizens' rights in governance, and bringing transparency through RTI, MGNREGA, food security and other legislation, large investments in social sector and a quantum leap in infrastructure investments in the economy."
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Former foreign secretary Shyam Saran, who was closely connected with the negotiations of the India-US civil nuclear deal, said Singh would be "remembered as one of India's most cerebral and visionary leaders who carried his wisdom and intellectual brilliance with rare modesty."
After the 2009 mandate, with the obstructive Left parties out of government, things should have looked up for the prime minister in the second term. Instead they deteriorated.
The chipping away at the position of the Prime Minister by the other centre of power that existed, Sonia Gandhi who wielded the upper hand, led to the unravelling of UPA 2, according to many.
According to one of his technocrat advisers, the turning point for Singh in his second term came when he underwent his second quadruple bypass surgery in 2009.
"Thereafter, he seemed to be giving up. He lost his drive and he became completely non-confrontational and was unwilling to intercede in ministerial and official disputes," the adviser told IANS, not wishing to be identified.
Another official at the PMO, who was privy to cabinet issues, said in more instances than one, when ministers came to him to resolve policy disagreements between them, Singh asked them to "resolve the problem between themselves" and refused to intervene.
He even told a senior government functionary, who suggested that he talk to the media more often, that he talk to the media on his behalf.
"He just seemed to be giving up at one time," recounted the aide.
In his latest blog, BJP leader Arun Jaitley sums up Manmohan Singh's position: "...He was literally a Prime Minister announced by Sonia ji. He had to function within that limitation."
He adds: "...He never wanted to rock the boat. He knew that he was vested with limited power and on all major decisions he had to keep the party and its first family in good humour."
BJP spokesman Prakash Javadekar told IANS: "Manmohan Singh was a good man but, unfortunately, he was the head of a bad government. He allowed the demeaning of his own office, he failed to stop the country from being looted by his ministers, and was also responsible for policy paralysis."
But V Narayanasamy, minister of state in the Prime Minister's Office (PMO), said the Prime Minister "will leave the legacy of a good administrator".
Calling Manmohan Singh an "upright and honest PM", Narayanasamy said he saw "the merit of every case before taking a decision".
On the opposition charges that the PM was "weak", and that this led to a "dual power center", Narayanasamy said it was "wrong criticism".
"It is on the basis of the party's policy that the government came to power. The party matters and it cannot be ignored. The party's ideology drives the government and both are interlinked," the minister told IANS.
The prime minister's communication advisor Pankaj Pachauri said that among his biggest acts was he kept the economy on good trajectory - but for the past two years when the economy was affected by the global downturn - and has written a new paradigm of inclusiveness.
Describing the Prime Minister, Pachauri told IANS: "I have never seen him angry. He has a fine sense of humour. He is very down to earth and has good equations with all ministers and colleagues."
According to Baru, Congress vice-president Rahul Gandhi's outburst last year when he publicly trashed an ordinance to save convicted legislators from disqualification as "complete nonsense", was part of a larger plan to replace Singh by anointing Rahul Gandhi.
Baru had publicly said then that the prime minister should quit after the insulting remarks.
Former cabinet secretary TSR Subramaniam, who held the post during 1996-1998, feels Manmohan Singh was "a good man" and was "chosen specifically to play second fiddle".
"My own judgment is that he was not cut out for that job. Leadership's job is to take decisions. The prime minister is not supposed to be number two. I think it (the post) was too big for him," Subramaniam told IANS.
"It is also important that the top man leaves the post in a better condition. He (Manmohan Singh) has diminished that office. The next prime minister will have to bring it back to its original (stature) and it is a difficult (task)."
(With PTI and IANS inputs)