Launched by journalist Mike King in 1992, Johnson King now has offices in the UK, France and Germany. It focuses particularly in the B2B sector, with clients including Silver Peak, Gigamon, and Kaseya.
King said his company had been in talks with a number of agencies about a potential deal over the past year.
He added: "We talked with a number of firms, and there is no doubt that Finn Partners was the best fit. It was important for us to find a partner that placed the same emphasis we do on the work environment, and on enabling people to express themselves and be creative."
Since then it has acquired Washington DC-based Widmeyer Communications and parts of New York agency Gabbegroup as well as opening a Finn Partners office in London.
The London office has developed a roster of clients covering consumer, education, social issues and tech.
Johnson King will continue to operate under its existing name, as a Finn Partners agency.