The State Department blocked Sondland from appearing before three House panels last week as the White House informed Democratic leaders they had no intention of cooperating with the impeachment inquiry.
U.S. Deputy Secretary of State John Sullivan arrives at the Lopez Palace, in Asuncion, Paraguay, on September 6, 2019.
Democrats have accused Mr Trump of withholding USA aid while urging Ukraine to investigate widely debunked corruption claims against his rival, 2020 Democratic frontrunner Joe Biden and his son, Hunter, who worked with a Ukrainian gas company.
Sondland was subpoenaed as part of the House Democrat's impeachment inquiry that is centered on a whistleblower complaint detailing concerns about Trump's conduct during a phone call with Ukrainian President Volodymyr Zelensky in July.
In her remarks, she said that the State Department had been attacked and hollowed out, and she predicted harm to the nation's interests.
She said she did not know Mr Giuliani's reasons for attacking her.
Trump had said in August he was considering Sullivan for the envoy job. Giuliani has said he provided information to both Trump and the State Department about Yovanovitch, who he suggested was biased against Trump.
She arrived on Capitol Hill on Friday despite the White House's stated objections and refusal to cooperate with the Democrat-led proceedings.
'Incredulous': Yovanovitch says she was "incredulous" she lost her post "based, as far as I can tell, on unfounded and false claims by people with clearly questionable motives", per the New York Times.
Trump recalled Yovanovitch from her US ambassadorship to Ukraine last May after unfounded claims she sought to undermine his policies.
"The president has been crystal clear no quid pro quo of any kind", Sondland said.
Miller musings: White House aide Stephen Miller declined to tell reporters whether he would comply with a congressional subpoena, if ever presented one.
The resignation this week of Michael McKinley, a career diplomat and senior adviser to Pompeo, came amid rising dissatisfaction inside the State Department at what is seen as Pompeo's failure to defend his subordinates who became targets in the Ukraine controversy. According to a cached biography on the State Department, earlier in her career she worked at the USA embassies in Ottawa, London, Moscow and Mogadishu. Also scheduled to testify next week are Deputy Assistant Secretary of State George Kent, State Department counselor Ulrich Brechbuhl, and U.S. Ambassador to the European Union Gordon Sondland.
Sondland's attorneys said that he does not have the power to share documents related to his official responsibilities sought by the committees because they are the property of the federal government and that it will be up to the State Department to do so.
President Donald Trump and Ukrainian President Volodymyr Zelenskiy looks on during a meeting in NY on September 25, 2019, on the sidelines of the United Nations General Assembly. At his session earlier this month, Volker turned over a series of text messages he exchanged with Giuliani, a top aide to Ukrainian President Volodymyr Zelensky and senior USA diplomats including Gordon Sondland, the US ambassador to the European Union.
Democrats are also expected to ask Ms Yovanovitch about the Trump administration's Ukraine policies, and about how Mr Giuliani and his associates had pressed Ukraine to investigate Mr Biden ahead of the 2020 presidential election.