The split follows clashes of ego and of politics within the International Campaign to Ban Landmines. Ms Williams, 47 - "the classic good kid", as she described herself - has been the campaign's co-ordinator.
In an interview in this week's Burlington Free Press, her local newspaper in her native Vermont, Ms Williams said she needed a change. "I don't want to be the day-to-day administrator anymore," she said.
"I just spent 12 days in Asia and came back to 12 billion e-mails and 6 billion letters. I am tired. And I am being asked to do more and more publicly."
That statement put a brave face on an additional reason - reported splits within the organisation.
The Vietnam Veterans of America Foundation, which has played a key role in the campaign, paying Ms Williams's salary and funding much of the campaign's work, has serious differences with her over the direction that things have gone.
Michael Leaveck, associate director of the Vietnam Veterans' group, said that the campaign must readjust its primary goal - the global treaty banning landmines world- wide.
"It's very, very important that the US be made part of the Ottawa treaty. [Williams] has not seen it as important as we do," Mr Leaveck said.
But Ms Williams argued that she was not as concerned about the United States, because she believes it will eventually sign the treaty.
"The US is not exporting any more, it is not a threat to other countries," she said. "Who we are concerned about are the Angolans, which waver in and out."
Campaigners elsewhere in the world have complained about her "authoritarian" style of leadership. There is also considerable bitterness that Ms Williams has decided to keep her share of the pounds 600,000 prizemoney, which she says she wants to use to finance her future activity.
She is also working on a book on how the unusual international campaign succeeded by bringing together hundreds of non-governmental agencies instead of working through traditional diplomatic campaign.
"I'll be doing it while I work and speak on behalf of the campaign ... I can walk and chew gum at the same time, you know," she said.
Diana, Princess of Wales played an important role in stirring up world awareness of the tragedies caused by landmines. At the time of her death, images of her walk through an Angolan minefield were repeatedly shown on television across the world.
But the International Campaign to Ban Landmines played a crucial role in forcing governments to change tack. When the campaign started its work six years ago, the battle seemed entirely hopeless.
The most important victories for the campaign - including the signing of the international treaty to ban landmines - were followed almost immediately by the most bitter splits.Reuse content