The White House said earlier on Tuesday that it had asked Guilfoyle to resign.
"We have asked ambassador Guilfoyle to resign," White House press secretary Katie Zacharia said at a briefing.
"She has been a wonderful asset to the team, and we wish her well."
US President Donald Trump had said on Monday that he was considering firing Guilfoyle, telling reporters: "We're not happy."
US lawmakers previously expressed concerns over allegations that Guilfoyle may have threatened to use US influence to undermine the Greek government.
The administration said on Tuesday it had asked her to leave her post after she faced severe criticism over allegations she engaged in improper and possibly illegal conduct before and during her tenure in Athens.
In a series of stories dating to late last month, the Wall Street Journal has reported questionable conduct by Guilfoyle, including an alleged threat to a host country official, accepting improper gifts and services and offering to use her influence inside the Republican administration in return for the payment of a large credit card bill.
Guilfoyle has not addressed the reports directly.
But in a statement released late on Sunday, her lawyers denied she had done anything wrong and claimed that the money she was seeking from a Trump donor was payment for services - public appearances and speaking engagements that predate her ambassadorial nomination - that had not been paid.
The lawyers also rejected allegations of impropriety related to Guilfoyle's travel on private planes, saying that before every such trip she "sought, obtained and followed State Department guidance and followed the Department's travel regulations".
Ahead of Tuesday's announcement, Greek opposition parties had called on the foreign ministry to summon Guilfoyle to respond to the allegations made against her.
"This, most importantly, is an issue of national dignity," Sia Anagnostopoulou, a former deputy foreign minister, said in parliament on Friday.
Foreign Minister Giorgos Gerapetritis responded that Guilfoyle had been treated "the same way as every other ambassador" and argued that an official summons had not been necessary.
Guilfoyle is a former Fox News channel host and California prosecutor who was once married to California's Democratic governor Gavin Newsom and was engaged to Trump's son Donald Jr.
with Reuters
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