Orlando Hooper-Greenhill, the TBWA\London planning partner, will join the pair as the third and final founder of the new agency. They will hold equal equity.
It remains unclear what the agency will be called, following news last week that Wnek has been blocked from using his name by Havas, his former employer. One option is for it to be called LWH-G.
Langdon said: "We were struck by Hooper-Greenhill's strategic and creative brains. He also has youth on his side, despite being grown up in his approach to business."
Hooper-Greenhill joined TBWA, where he worked on Abbey, Scottish Courage and Eurostar, at the end of 2001. He moved from DFGW, where he spent a year. Before that, he was at Leagas Delaney for three years. He also spent six years working in research, primarily at Hall & Partners.
He said: "Ben and Mark are two of the most charming and energetic guys I've ever met. Their reputation is a bit like a ghost story: half of it's true and half of it's what people want to hear."
TBWA's planning director, Neil Dawson, said: "It's a great opportunity for Orlando. We wish him well."