Motorola Mobility will move its Libertyville headquarters — and 3,000 workers — to downtown Chicago, taking the top four floors and rooftop of the Merchandise Mart and becoming the landmark building’s largest tenant with 600,000 square feet, Motorola Mobility CEO Dennis Woodside and Mayor Rahm Emanuel announced Thursday.

Motorola Mobility, the cellphone and TV set-top-box divisions of the old Motorola Inc., was bought by search giant Google for $12.5 billion on May 22, and is run out of Google’s headquarters in Mountain View, Calif.

But Mobility executives pledged a year before the Google takeover to keep Mobility’s well-paying engineering, finance, marketing, design and executive jobs in Illinois so Mobility could benefit from statewide tax credits worth more than $100 million over a 10-year period.

Gov. Pat Quinn said at a news conference in Deerfield that he gave Google “permission” to move from Libertyville to downtown Chicago, since that was the location Google preferred.

Motorola Mobility will join 1871, the Chicagoland Entrepreneurial Center’s 50,000-square-foot incubator for technology startups, and tech-related companies such as Allscripts and Razorfish in the Merchandise Mart, creating what Emanuel termed a “tech campus” for the city.

Mobility will be the driver in making Chicago “the digital capital of the Midwest,” Emanuel said.