The 77-year-old 'Raja of Manda', a sobriquet he earned because of his origins in the principality of Manda in Uttar Pradesh's Fathepur, entered politics in Allahabad during the Nehru era and soon made a name for his rectitude.
He earned the title of 'Mr Clean' despite occupying positions of power, including the Chief Ministership of Uttar Pradesh which he had resigned in the early 80s when his brother was killed by dacoits, and as Minister at the Centre.
Singh resigned as Defence Minister after he was shifted from Finance in 1987 at the height of his campaign against leading industrialists on the issue of tax evasion and later took on the then Prime Minister Rajiv Gandhi by quitting Congress on the issue of Bofors scandal.
Forming Jan Morcha, an amalgam of disgruntled Congressmen, he later became the pivot around which the opposition came together to dethrone Congress to give the first non-Congress coalition at the Centre, supported by the Left parties and the BJP from outside.
At the height of his political fight with Gandhi, Singh faced the 'dirty tricks' department of the government which allegedly sought to sully his name by fabricating illicit accounts in the name of his son Ajeya Singh in tax havens abroad.
Singh, an upper caste Thakur, surprised the nation by introducing the concept of reservation in Central services by an executive order, an issue that was hanging fire for decades under the earlier Congress regimes.
His critics would argue that he played the reservation card when he was faced with political challenge from the late Congress leader, Devi Lal, who was the deputy Prime Minister in the fractious Janata Dal-led National Front Government he headed for 11 months from December 1989.
That Devi Lal joined Singh's detractor Chandrasekhar to topple his government with help from Congress after BJP withdrew support to him on the issue of Ram Janmabhoomi movement.
Singh also heralded the concept of outside support and brought sworn enemies -- Left and BJP -- into an informal Coordination Committee that ran his government.
His coalition also brought regional parties like DMK, Telugu Desam and Asom Gana Parishad into the Union Government, a trend that has continued ever since except during the five-year term of late P V Narasimha Rao.
Educated in Allahabad and Poona Universities, Singh took his BA, B.Sc and LLB degrees and became a member of the Uttar Pradesh Assembly in 1969. He was elected to the Fifth Lok Sabha in 1971 and became Union Deputy Minister of Commerce and later Minister of State for Commerce in the Indira Gandhi Government.
From the Lok Sabha, he was sent as Chief Minister of Uttar Pradesh by Indira Gandhi in 1980, which he had quit two years later again to become a Union Minister under her.
Even his election as Prime Minister in the Janata Dal parliamentary party meeting after the 1989 polls was marred by controversy when Devi Lal refused to accept the leadership under an 'arrangement' that irked Chandrashekhar, who refused to join his cabinet.
Singh's Government faced early problems when Kashmir erupted in flames on cries of 'azadi' (freedom) and his government released militants in exchange for the safety of Rubaiya Sayeed, the kidnapped daughter of his Home Minister Mufti Mohammed Sayeed.
He also sent Jagmohan as Governor of Jammu and Kashmir and had to recall him when feelings were inflamed after the Governor ordered troops to fire on the funeral procession of Mirwaiz.
However, in another troubled area -- Punjab -- Singh replaced hardline Siddhartha Shankar Roy with a former bureaucrat Nirmal Kumar Mukherjee, who planned early elections.
He also made a much-publicised visit to Golden Temple to attorn for 'Operation Blue Star' and Punjab markedly saw improvement in the situation since then. He also withdrew the Indian Peacekeeping Force (IPKF) from Sri Lanka.
Even after he lost Prime Ministership, Singh continued to be a mentor for the non-Congress forces but towards his later days he made peace with the grand old party and with Rajiv Gandhi's widow Sonia.
A landlord-turned-fierce champion of oppressed classes, a poet and a painter, Singh was attracted to Bhoodan Movement of Acharya Vinoba Bhave and donated land to poor peasants in 1967.
Away from politics, Singh had always taken a keen interest in poetry and painting. He had penned a number of poems and his first anthology of poems published was 'ek tukra dharti, ek tukra aasman' (a piece of land, a piece of sky). His last poem was published a few years ago.
"There is a certain magic in lines -- whether they be in painting, prose on poetry," Singh had said on the occasion of the release of his first collection of verses.