Thackeray, along with top officials of Mumbai Metropolitan Region Development Authority and others launched the girder at the Mumbai-end of MTHL in Sewri.
The girder weighing 1,000 tonnes was set off by a 1,400-tonnes 'launching girder' and mark the erection of the first span in the civilian works of the MTHL.
The MTHL, the second road link cutting through the Arabian Sea in the island city's eastern coast, will connect the mainland with Sewri and Chirle respectively.
It is expected to be completed by 2022 and slash the travel time between Mumbai and Navi Mumbai from existing 100-odd minutes to barely 30 minutes.
Estimated to cost around Rs 14,260 crore, it is being financed by a loan from the Japan International Cooperation Agency (JICA).
With around "19 percent financial progress" achieved, the MTHL project is being executed in three packages by three contractors with one intelligent transport system since April 2018.
The work on the six-lane plus two emergency lanes, 27-metre wide bridge will attract over 70,000 vehicles plying on it daily when thrown open to traffic.
The total bridge will be 22 kms long of which 16.5 kms will run in the sea, with interchanges on both sides, making it the longest sea-bridge in India, and second after the Rajiv Gandhi Bandra Worli Sea Link, which is now being extended up to Andheri.
The MTHL will be connected to the new international airport coming up in Navi Mumbai, the Jawaharlal Nehru Port Trust, the country's first expressway, the Mumbai-Pune Expressway, Mumbai-Goa Highway and provide faster access to north Maharashtra and southern India.
The MMRDA will install 'noise' and 'vision' barriers on a 6-km stretch of the MTHL, first (vision) to block the view of the sensitive BARC nuclear complex and the other (noise) to protect the movement of Flamingos and other migratory birds at the Sewri mudflats, besides silence zones on both sides.
The work of piling and pier construction is currently underway along with constructing a temporary bridge with casting of segments.
Conceived over 45 years ago, the project was finally cleared by former Chief Minister Prithviraj Chavan in 2012 and after securing all clearances, Prime Minister Narendra Modi had laid the foundation stone for the MTHL in December 2016.
( With inputs from IANS )