Marsh, a consultant and former international affairs officer for the Archbishop of Canterbury, will take over on a three-month interim basis from Intelligent Giving's more outspoken director Adam Rothwell in July. Rothwell is departing to begin a new career as a history teacher (Third Sector Online, 28 May).
Marsh says he was surprised when he got the call. For observers, his appointment is equally surprising: although the Impact Coalition and Intelligent Giving are both about improving transparency, the similarities end there.
The coalition was set up by charities in 2005 to explain to the public how charities work. Sector outsiders - journalists - launched Intelligent Giving in 2006 to help the public decide which charities to donate to.
Intelligent Giving is good at making national headlines that provoke charities, most recently with its call to boycott chuggers.
The site became unpopular with the Institute of Fundraising, which until recently was the Impact Coalition's main funder. Megan Pacey, the institute's former head of policy, described Intelligent Giving's staff as "self-appointed charity watchdogs" and its research methods "crude".
Marsh says that, despite his paymaster's public sparring with Intelligent Giving during his time at the coalition, he regarded Rothwell behind the scenes as not only a friend, but also an understanding and supportive intellectual adviser.
"We talked and argued quietly on many issues," he says. "If I thought a particular charity was lacking transparency, I would call Adam. We'd have long conversations about the principles involved."
Rothwell says Intelligent Giving's clashes with the institute and other fundraising organisations such as the Public Fundraising Regulatory Association have been in donors' interests. "The clashes have always been with the purpose of providing a service to donors," he says.
In the coming months, Intelligent Giving will try to position itself as the charity sector's version of the consumer organisation Which? by overhauling its rating system to tell donors which charities it thinks are best, as well as which are transparent.
Marsh backs this approach. He says Intelligent Giving will be "the place where consumers will come for good, reliable information about charities".
"I'm big enough and nasty enough to cope with criticism, and it will be great to have the freedom to say some of the things I've been thinking," he says.