The 190,000 sq ft building has been designed by FaulknerBrowns Architects as two blocks linked by a glazed atrium.
It represents phase two of the redevelopment of the old 5.5-hectare Vaux Brewery site in the city centre and is twice the size of the first building that has been put up, called The Beam, which was originally started by Carillion and completed by Tolent in June.
Bowmer & Kirkland is scheduled to complete the new City Hall building in autumn 2021.
Project manager Paul Anderson said: "We are delighted to be the main contractor on this important building for the city of Sunderland. This development will herald the regeneration of the city centre and will also encourage further development and investment, boosting the local economy."