COH founder and managing director Chris O'Hare will join the First Protocol board as a director and shareholder, and the rest of the COH team will transfer to First Protocol's Waterloo premises.
COH's skills, which include creating event concepts, design, web development and advertising, will now become part of First Protocol's offering.
First Protocol said that the acquisition formed part of the its five-year plan to grow both its market share and global footprint and comes less than half a year after it opened its Los Angeles office.
Richard Waddington, First Protocol’s chief executive, said: "We went into the recession with no debt and a strong balance sheet; two years on we are now in an even healthier position.
"We have expanded our client base and doubled the size of the business, while this acquisition brings greater design capabilities in-house, both in terms of traditional and digital media, without weakening our financial stability.
Waddington added that the agency is now in profit for the year ending September and has doubled headcount - from around 30 staff last year to 70-plus people.
First Protocol said that while COH becomes part of the agency, it will retain its brand identity.