Earnie founder and MD Alistair Gammell remains with the combined business and takes a stake in Threepipe - there was no cash involved in the transaction.
Gammell set up Earnie in 2015 as a spin-out from b2b agency Earnest, and recently bought out the business' other partners.
Earnie now has 12 staff - a mix of creatives, strategists, animators, designers and producers - bringing Threepipe's total headcount to circa 90. Threepipe grew by more than a quarter last year, bringing in total revenues of £5.1m, the PRWeek Top 150 shows.
Threepipe had been working with Earnie on mutual clients such as the England & Wales Cricket Board since its launch, according to Threepipe co-founder Jim Hawker. Other clients of Earnie include the NFL, Monster Energy, Cricket West Indies, The Jockey Club and UK Sport.
"We have worked with Earnie on joint campaigns and admired their expertise from afar. As more of our campaigns become more visual through multiple channels, it’s key that we have the right combination of strategic approach and creative capability in-house," said Hawker, adding that there was a "good personality fit" between the businesses.
Gammell said: "Being part of Threepipe will help us meet the growing demand for measurable content campaigns on an international scale."
The news comes two years after Threepipe acquired 12-person SEO agency Spot Digital. As with the Earnie deal, Spot's founder Tony Thomas also stayed with the business as a part-owner.