The 59-year-old is set to lead a government of the Shiv Sena-Nationalist Congress Party-Congress alliance, called the Maha Vikas Aghadi.
This is also the first time that the Shiv Sena is joining hands with the Congress and the NCP - once bitter adversaries of the regional party founded in 1966 by Uddhav's father Balasaheb Thackeray.
"I had never dreamed of leading the state," Uddhav Thackeray said after being elected to lead the coalition. Images from the Sena first family's residence Matoshree, showed him kneeling in front of a photograph of Bal Thackeray.
The Sena chief has never contested an election and is not a member of either of the House of the state legislature.
His son Aaditya became the first member of the Thackeray family to contest an election when he won from the Worli assembly constituency last month.
According to sources, Uddhav Thackeray is likely to become a member of the Legislative Council like Congress's Prithviraj Chavan did when he took charge of Maharashtra after Ashok Chavan's resignation as Chief Minister in 2010.
Born on July 27, 1960, Uddhav Thackeray took charge of the party after Bal Thackeray died in 2012.
As the third of the firebrand Marathi politician's son, low-key Uddhav was not seen as his political heir.
A passionate photographer, he seemed more at ease shooting the tiger - his party's symbol - than riding it.
He started his political journey when he began handling the daily business of the Shiv Sena's mouthpiece Saamana.
He openly criticised the administrative skills and working style of Narayan Rane, who was the Sena chief minister of Maharashtra in 1999 and the tussle led to Mr Rane's resignation. Mr Rane was expelled from the party as well.
Uddhav Thackeray led the Shiv Sena to a massive victory in the Brihanmumbai Municipal Corporation elections in 2002.
In 2003, he was made the party working president. After a fallout between him and his cousin Raj Thackeray in 2006, the latter went on to form the Maharashtra Navnirman Sena.
Bal Thackeray's death in 2012 formalised Uddhav's hold over the Sena, but his party's political fortunes continued to wane.
Uddhav Thackeray's own political future seemed uncertain: a cardiac surgery in 2012 is believed to have slowed him down.
In 2014, the Shiv Sena broke up with the BJP but ended up, for the first time, as the junior partner in the alliance.
Uddhav Thackeray doubled down on backing the rise of Aaditya, who steered the Sena in a direction set by his father: as a less aggressive, more urbane political force.
As the Sena's ambitions clashed with a new BJP, the party assumed the role of Maharashtra's de-facto opposition despite being a member of the Devendra Fadnavis government.
Tensions peaked in the run up to the 2019 elections, when Uddhav Thackeray himself used Rahul Gandhi's infamous line in an election rally targeting Prime Minister Narendra Modi - "Chowkidar Chor Hai".
The BJP went on to sweep Maharashtra and India but the Sena had a chance to strike back after the assembly elections as the BJP's tally fell.
Against all expectations, the mild-mannered Uddhav Thackeray broke ties with the BJP and forged an alliance with his most bitter rivals. Standing his ground against voices of disagreement Uddhav refused to take calls from Devendra Fadnavis after he said no promise of sharing the Chief Minister's post on a rotational basis had been made to the Shiv Sena like Uddhav Thackeray had claimed. Uddhav fumed at being called a 'liar' and chose to begin talks with the NCP and Congress as the BJP opted against forming a government due to lack of numbers.
"I will go to Delhi to meet my 'elder brother' after the government is formed," he said on Tuesday, referring to PM Modi who had called him "my younger brother" during campaign rallies.
He also thanked Congress president Sonia Gandhi and NCP chief Sharad Pawar. "I want to thank Sonia ji also. Parties with different ideologies have come together...those who were friends for 30 years, did not trust us. But those against whom we fought for 30 years have trusted me," Mr Thackeray said.
Political analysts see it as a massive risk but one that improbably catapulted him to the Chief Minister's chair, making him the first Thackeray to give up the remote control for real control of India's richest state.