In an e-mail to all staff, which was leaked to Campaign, the Zulu chief executive, Martin Brooks, informed colleagues that Madden was leaving.
He wrote: "We've done our best to support him but, ultimately, he has decided that it's not right for him."
The e-mail also included a quote from Madden, who said: "I may or may not be direct marketing's answer to Thierry Henry, but even he can't score when he's not match-fit. I had a very stressful 2006, and I simply wasn't ready to take on a job of the scale of Claydon Heeley. I've admitted this, and I'm moving on."
As well as the role of chief executive, Madden also took on the position of strategy director at the Omnicom network Zulu when he was hired in January.
Madden resigned from Tequila\London, where he was the planning director, last autumn. At that time, Madden was in discussions with Mike Cornwell about launching a direct agency, which The Red Brick Road was going to back. He was also linked with plans for a Mother-backed start-up earlier in the year. Neither agency came to fruition.
Madden was a former creative director of GGT Direct. Following its merger in 1997 with TBWA, he became the planning director of the merged agency.
Brooks added: "We have enjoyed having Richard in the agency for seven weeks, but it is clear that he needs time and space to work out what's best for him in the long term. We would like to wish him all the very best."