Marash was the highest-profile signing when the English-language affiliate to Al-Jazeera was started two years ago in an attempt to compete with CNN and the BBC.
He said there was a "reflexive adversarial editorial stance" against Americans at Al-Jazeera English.
Marash said: "Given the global feelings about the Bush administration, it's not surprising."
Marash is being being replaced by former CNN International host Shihab Rattansi and he said that there were now more Canadians than US citizens working at the Washington Bureau.
Will Stebbins, Washington bureau chief for Al-Jazeera English, denied any bias against Americans.
He said: "We certainly evaluate US policy rigorously. But we do our best to give everyone a fair shout."
Marash joined Al-Jazeera from ABC News where he worked on 'Nightline'. He also anchored newscasts at WNBC in New York and WRC-TV in Washington DC.
Last week, Steve Clark, a former senior executive at ITN and Sky News who spearheaded Al-Jazeera English's launch in November 2006, also resigned.
The channel has been the subject of speculation in recent months, with speculation of a split between the management of the original Arabic channel and those in charge of the English-language spin-off. There are complaints that the newcomer has dulled Al-Jazeera's focus and sapped resources.
Meanwhile, more than 15 staff are said to have quit the English-language channel's headquarters in Doha, Qatar, in recent months.
When the channel launched it made a number of high-profile signings including BBC reporter Rageh Omar and Sir David Frost, who interviewed Tony Blair in the channel's opening week.