PAN, which generated revenue of $19.9m (£15.9m) in 2018 with a headcount of 134, has acquired 100 per cent ownership of the business for an undisclosed sum.
The move will see Capella rebranded as PAN UK. Co-founder Gareth Thomas becomes PAN UK managing director, "responsible for providing sound strategy to clients that are looking to expand internationally, strengthening engagement with agency partners in key EMEA regions and deepening PAN’s roots in Europe".
Fellow co-founder Nia Evans becomes vice president and general manager, focused on client service and the overall employee experience.
The London agency, which launched in 2012, is to move office in the next couple of months to accommodate growth. It follows recent account wins including Samsung, on a cross-category, hybrid corporate/consumer brief; and Spond, which is a Norwegian app aimed at helping grassroots coaches manage teams.
PAN and Capella have worked together on several accounts in the past, including cloud communications firm 8x8 and software company Actian.
Thomas told PRWeek: "If I'm perfectly honest we weren't looking to sell. We were in really good shape. We knew [PAN] anyway because we shared a number of clients. From our perspective we were looking to scale up our international capabilities, particularly in the US. They came along and said they were looking to get a foothold in the European market, the EMEA market and would you be interested?
"We thought we probably will [sell] at some point and these are really nice people who share the same kind of values, the same kind of way of working, so let's do it."
Thomas said he was also attracted to the fact PAN operates in healthcare comms. "That's really good for us because we've dabbled in a bit of healthcare where it overlaps with technology. That's an element that we're looking to get into as well."
Asked about an earn-out period, Thomas said: "Nia and I are not going anywhere. We are contracted to the business for many years and that suits us perfectly."
Independently-owned PAN, which was founded in 1995, has offices in Boston, New York, San Francisco and Orlando.
The company made its first acquisiton in 2015, snapping up San Francisco's Vantage PR. The Capella deal is part of the second half of its five-year growth plan.
President and CEO Philip A. Nardone Jr said: "A global acquisition has been a part of our plan for several years, and we are thrilled to have identified and acquired a firm with such remarkable partnership potential.
"I’ve always taken great pride in leading an organisation that cares about its people first and foremost, and Capella embodies this approach. I have total confidence in Gareth and his leadership team, as we share similar values and have a strong commitment to building a culture-first organisation. I envision this new chapter to be marked with excitement and opportunity for our clients and employees as we continue to strategically evolve the PAN story."