Often referred to as the Chanakya of Bihar politics, Mr Kumar, 66, quit as the CM of the politically crucial state last evening and readily accepted BJP's support to continue in power, a move that could have a bearing on the 2019 Lok Sabha polls as it is seen as a major setback for efforts to forge opposition unity to stop the BJP juggernaut.
Mr Kumar led the 'Mahagathbandhan' (Grand Alliance) that comprised his JD(U), the RJD and the Congress to a stunning victory few thought possible after being bruised and bloodied in the 2014 Lok Sabha polls. Mr Kumar, who first became the chief minister on November 24, 2005, was sworn in for a fifth term on November 20, 2015.
For the cool, suave and workaholic Nitish, he knew his association with Mr Prasad, 69, whom he called his 'Bade Bhai', would be the main point the BJP-led opposition could always score off, but he countered it with an answer to anything and everything thrown at him.
Soft-spoken, sharp and articulate, Mr Kumar, who speaks in a measured tone, had an answer ready with a subtle fact that he himself "had been with the BJP but never been coloured, and in any case Lalu is and remains a socialist."
Persuasive, sometimes adamant, Mr Kumar is known to carry strong likes and dislikes and never forgets a slight--a trait for which Mr Prasad described him as having 'aant mein daant' (teeth in his belly).
Working with his new tactician Prashant Kishor, a key strategist for the BJP in the 2014 Lok Sabha polls, Mr Kumar worked a carefully crafted soft campaign template for Bihar in the 2015 Assembly elections.
He invoked only his work, the potential of the next government with him, the slights to Bihar and his own self, strident appeals to Bihar's 'Asmita'(pride) and speaking for himself as the lone Bihari against the 'Bahari' (outsider), a veiled reference to Prime Minister Narendra Modi.
Even Sushil Kumar Modi, the tallest BJP leader from Bihar and once his deputy and friend, acknowledges Mr Kumar to be a visionary who changed Bihar.
His breakaway from the BJP in June 2013 stunned the nation, as did his resignation from the chief minister's post after the JD(U)'s massive Lok Sabha defeat in May 2014 and elevation of his protegee Jitan Ram Manjhi as chief minister and then pulling the latter down to assume power again in February 2015 for a fourth term.
Mr Kumar is also widely known as the 'Sushashan Babu' (Good governance man) and 'Vikas Purush', whom even critics credit with turning Bihar around.
Following the age-old proverb enemy's enemy is friend, the JD(U) leader whose party won just two of the state's 40 seats in the Lok Sabha poll, joined hands with Prasad to halt bete noire Narendra Modi's juggernaut in Bihar in the 2015 Assembly polls.
Mr Prasad and Nitish, friend-turned-foe in state politics, sank their differences to revive an alliance that began over 40 years ago with the students' agitation, which soon turned into a pan-India movement led by veteran socialist leader Jayaprakash Narayan.
Though Mr Prasad got lucky in his very first outing in the electoral arena, winning Lok Sabha poll in 1977, it took Kumar, an electrical engineer from NIT Patna, then known as Bihar College of Engineering, eight more years to get elected to the state Assembly for the first time in 1985, after having lost twice.
Though as different as chalk and cheese, Mr Kumar backed Prasad in bagging the chair of the Leader of Opposition in the assembly in 1989 and again when he challenged Ram Sundar Das and Raghunath Jha, nominees of Prime Minister V P Singh and Chandra Shekhar respectively, for the chief minister's post after Janata Dal came to power in Bihar in 1990.
Mr Kumar, who won the 1989 Lok Sabha polls from Barh, shifted his focus to Delhi, getting elected to the lower house in 1991, 1996, 1998 and 1999. He became Minister of State for Agriculture in the Atal Bihari Vajpayee government and then briefly the Railway Minister in 1999. He resigned after the train tragedy at Gaisal in West Bengal in 1999 in which nearly 300 people were killed.
Mr Kumar again became Railway Minister in 2001 and continued till 2004 during which he was credited with introducing several reforms in the public sector behemoth like Internet ticket booking and Tatkal system of instant booking.
The Godhra train burning incident in February 2002, which provided the spark that led to communal riots in Gujarat, occurred during his tenure at Rail Bhavan.
The JD(U) stalwart reworked his social engineering strategy and made a rainbow coalition of OBCs, EBCs, Mahadalits and minorities that helped him sweep Bihar in November 2005 elections, thereby uprooting 15 years of uninterrupted Lalu-Rabri rule.
Bihar became a completely 'dry state' last year banning sale and consumption of all kinds of liquor in a move seen as fulfilment of a poll promise to women by Mr Kumar. Mr Kumar's passionate pitch for prohibition citing the adverse effects of domestic violence and broken families besides economic and health costs had earned overwhelming support from women for the ruling JD(U) in the 2015 Assembly polls. He reiterated his commitment while announcing the ban.
Mr Kumar's backing to BJP presidential candidate and former Bihar Governor R N Kovind by breaking ranks with the opposition and his support of the Modi government's demonetisation drive had triggered speculation that he was warming up to the party.