Investment firm the Stagwell Group, set up in June by PR and public affairs veteran Mark Penn, has acquired SKDKnickerbocker in its first purchase.
Founded in 1968 by Bob Squier, SKDK has offices across the US in Washington DC, New York City, Los Angeles and Albany.
The PR agency, specialising in public affairs and political consulting, will continue to run as a standalone firm with the same management and executive.
Josh Isay, who will become managing partner of SKDK, said the firm is "excited about this new chapter" in its history.
"The ability to work with the Stagwell team and the companies they will have under their umbrella will enhance services for our clients, particularly in the growing fields of data analytics and digital innovation," he said.
"At SKDK, we know that the long-term success of our company is predicated on our commitment to excellence on behalf of our clients and being the home of a world-class team of communications professionals. With today’s announcement, we are renewing that commitment."
Stagwell’s primary backer is former Microsoft head Steve Ballmer, who has contributed US$250 million to the fund.
In a statement, Penn said: "Stagwell is pleased to welcome SKDKnickerbocker as its first investment. They are a terrific and talented firm in the critical growing area of strategic corporate and political communications. They have great potential to grow in the digital age, and we love the passion the leadership and their teams bring to their clients and causes."
Penn said the company may look to leverage debt and make further acquisitions of up to US$750 million.
In an interview with The New York Times, Penn spoke of the SKDK purchase being the first step in creating a "great group" of communications firms "that have a strong understanding of the digital world".
"It’s a bit like Noah’s ark, but we’ll have one of each kind," Penn told the Times.