The new City Hall is being built on the site of city's former Vaux brewery site, spearheading a wider £500m regeneration of the city centre.
Bowmer & Kirkland won the construction contract last month and is expected to complete in autumn 2021.
City Hall will house not just Sunderland City Council offices but also public sector partners including housing provider Gentoo and the Department for Work and Pensions.
Designed by architect Faulkner Brown, the building will span 190,000 sqft, making it more than twice the size of The Beam, which was the first building to be constructed on the former Vaux site.
Council leader Graeme Miller said: "City Hall is a real game changer for the regeneration of Sunderland city centre and I'm delighted to see it going ahead. Its development is an essential element of our £500m transformation plans to create a dynamic, healthy and vibrant city centre and deliver the step change that we know our residents want to see.
"It's also a further indication of the real sense of momentum building in Sunderland, with so many new developments either planned or underway and a renewed sense of confidence in the city."
Bowmer & Kirkland project manager Paul Anderson added: "We have been looking forward to working with Sunderland City Council on this project and it is really good to now be on site, getting the project underway and continuing the regeneration of this area of the city centre."