Work started the week of April 1, as construction equipment from Clark Construction was put in place and protective fencing was installed around the building's perimeter and outdoor plaza, a news release on the project said.
The Thompson Center's Clark/Lake L station, where six train lines converge, will remain open throughout construction.
"As the transformation of the Thompson Center begins, I want to thank Google once again for reaffirming their commitment to Chicago and preserving this historic building while updating and modernizing this iconic space," Pritzker said in a statement. "The Google employees who will call this building home will be another piece in the complex, thriving puzzle of the Loop's infrastructure and ecosystem, making our economy and our skyline stronger."
Gov. Pritzker speaks on start of Thompson Center construction
SEE ALSO: Monument with Standing Beast, outside Thompson Center since 1980s, to be moved to state facility
Prime | Capri is leading the build-to-suit redevelopment project under an agreement with Google, which intends to purchase the building once renovations are completed, the release said.
PCI bought the Thompson Center in July 2022.
"As we begin construction on the Thompson Center redevelopment, I want to express our city's gratitude to Google and Prime | Capri Co-principals Michael W. Reschke and Quintin E. Primo III for their work in reimagining this iconic Chicago landmark as a beacon of innovation and sustainability," Johnson said in a statement. "This project brings numerous construction opportunities to our city while also preserving a vital transit hub, which reinforces our shared commitment to accessibility. And as a future home for Google, this site will undoubtedly stand as a symbol of Chicago's stature as an inclusive hub for business and technology on the global stage."
Google released renderings of the building late last year.
Chicago Mayor Brandon Johnson speaks on Thompson Center construction
The company is now working with the building's original architects to update it while maintaining its iconic form in a $280 million project.
The building was originally designed by renowned architect Helmut Jahn.
"This building put him on the world stage, and I remember watching this going up when we were in architecture school. We even took tours of this when it was under construction," said Ward Miller, executive director of Preservation Chicago.
SEE ALSO | Thompson Center architect Helmut Jahn killed in Campton Hills bike accident
The Thompson Center officially opened in 1985, and was home to the state of Illinois offices. As recently as 2020, Preservation Chicago had designated the Thompson Center as one of the city's most endangered buildings.
The building's futuristic structure was billed as one for the 21st century, but construction crews stripping the building's non-insulated glass and metal skin are starting the real work of creating a building for modern times.
"Our vision for a reimagined Thompson Center includes upgrading the building to an energy efficient LEED Platinum building, supporting Google's net-zero carbon goals," said Karen Sauder, Google Chicago site lead.
The site will also include new green spaces through covered terraces along its southeast perimeter.
"We knew the incredible potential of this space as a cultural epicenter, as transit hub, a central location," Pritzker said.
While many details are under wraps, preservationists who fought for years for the Thompson Center to receive landmark status are just grateful to not be looking at a hole in the ground.
"We see this as a win-win today. It's a re-investment in one of our most extraordinary buildings. It's bringing thousands of jobs to downtown, and we're seeing a special and monumental place still here for the public," said Bonnie McDonald, CEO of Landmarks Illinois.
Google is targeting 2026 to be operational at the site. Once finished, Google is expected to move in about 2,000 employees.