Motorola introduced the first mobile phone nearly 30 years ago, but has lately struggled with enormous pressure from competitors, including Apple and Nokia, in a business with swiftly evolving technology.
Motorola Mobility, however, split from the main company eight months ago and has seen revenues rise 28 percent in the second quarter.
Google touted Motorola's commitment to the open platform Android operating system, which it developed and which it says will remain open to other hardware companies.
"Today, more than 150 million Android devices have been activated worldwide," Google's Chief Executive Officer Larry Page said in an online posting.
"Given Android's phenomenal success, we are always looking for ways to supercharge the Android ecosystem," Page said.
Google's purchase price represents a 60 percent premium on Motorola's closing Friday price in the stock market.
Google, which expects the deal to close in 2012, will be beefing up its portfolio of patents just after losing out on a multibillion-dollar patent auction hosted by bankrupt Canadian phonemaker Nortel.