The integration of Atomic's six US offices will enlarge Grayling's presence across the Atlantic.
Atomic was acquired from co-founders Andy Getsey and James Hannon and a number of minority shareholders for $13.3m (£8.3m). Huntworth has reached a new agreement on their earn-out payments that defers a sum of £2.
Getsey and Hannon will have senior roles at Grayling US under recently installed chief executive Peter Harris, who was previously with MSLGroup.
Getsey will assume the role of executive vice-president, and Hannon will serve as Grayling's chief data strategist. They will also co-chair the Grayling US technology practice.
Grayling will now have offices in New York, Washington, D.C., Tallahassee, Denver, Seattle, Sacramento, San Francisco, Silicon Valley, Los Angeles and Orange County.
Pete Pedersen, chief executive officer of Grayling, said: "Integrating Atomic gives Grayling US additional critical mass in PR, social media and digital, as well as significant expertise in the use of data to guide strategy and creative. We're delighted to welcome Atomic into the Grayling group."
Getsey said: "We're very proud that the Atomic team accomplished its goal of becoming one of the best and most innovative mid-sized PR firms in the digital era. As Grayling, we look forward to helping create an innovative national and global multi-disciplinary communications powerhouse."
Grayling was founded in 1981 and specialises in data for digital marketing, public relations, government affairs and investor relations. It has 54 offices in 26 countries worldwide across Europe, North America, the Middle East and Asia.