3 Monkeys, set up by Angie Moxham 13 years ago, will not divulge a sales price, but experts predict the deal would be worth up to £15m ($21.5m), with around half the cash paid upfront.
The London agency had fee income of £6.1m in 2014, total revenues of £12m and employs more than 60 staff. Clients include United Biscuits and TGI Fridays. Managing director Christine Jewell is also a shareholder.
Further acquisitions in the region by Zeno, probably in different disciplines, are expected soon.
Zeno, which makes global revenues of $48m (£33.4m) annually, will merge 3 Monkeys with its UK operation to create 3 Monkeys | Zeno – or 3MZ – with 75 staff in the combined agency. Moxham, who is understood to be on a performance-based earnout agreement, becomes chief of Zeno Europe, reporting to Zeno's global chief executive Barby Siegel, who is based in New York and in turn reports to Richard Edelman, chairman of Zeno parent firm DJE Holdings.
Jewell will remain as MD, running the day-to-day UK operations, while Steve Earl, who has been managing director of Zeno UK since 2012, will assume the role of MD Zeno Europe. Earl's UK team is in the process of moving into 3 Monkeys' premises in Soho, London.
On 7 March, the 3MZ team will be bolstered by Tristan Pinero, former creative director and board member of Grayling UK and Ireland. He previously spent almost five years as a director at the firm, and will co-head 3MZ's consumer & brand practice alongside Anna Speight.
Zeno's global client base includes Merck, Netflix, AB InBev, Intel and Lenovo. The agency was set up in 1998 as PR21, originally as a 'conflict shop' to Edelman. But since 2010, Zeno has grown rapidly, from 60 people then to more than 400 today, including eight offices in Asia. Three-quarters ($35m) of the network's revenue still derives from the US, but Zeno aims to create more revenue from Europe. Siegel told PRWeek the aim was to create an agency with revenues of $100m.
Moxham said: "We have been looking at how we globalise 3 Monkeys for a few years now, in response to client demand, including us going on the acquisition trail. No route, however, could come close to the capabilities and bandwidth we gain on day one by joining forces with Zeno.
"We can now support current and future clients with our special monkey magic around the world, with people who think and behave in the same way as us. Importantly, we also remain independent."
Richard Edelman said the move "accelerates the global expansion of Zeno", going on to say: "Bottom line: these are two companies that were meant to be together."