Sondland was initially set to testify last week on the Hill, but his testimony was derailed when the State Department blocked him from appearing. The three Democratic committee chairs then issued a subpoena demanding Sondland turn over documents and appear for a deposition.
Lawmakers have been eager to press him about text messages he exchanged related to Trump's July phone call with Ukrainian President Volodymyr Zelensky and the freezing of foreign aid to Ukraine that cut to the heart of the Democrats' impeachment probe.
Sondland has been a player in Republican politics for a number of years but wasn't always a Trump supporter.
He was previously the founder and CEO of the Provenance Hotels chain, which boasts 19 hotels across the country.
Sondland was confirmed to the ambassador role on June 29, 2018.
"Mr. Trump's statements have made it clear that his positions do not align with" his personal beliefs and values, Provenance Hotels spokeswoman Kate Buska told the Portland newspaper.
"Historically, Mr. Sondland has been supportive of the Republican party's nominees for President," she added. "However, in light of Mr. Trump's treatment of the Khan family and the fact his constantly evolving positions diverge from their personal beliefs and values on so many levels, neither Mr. Sondland or Mr. Wali can support his candidacy."
Although the bulk of his donations have been to GOP candidates, he gave over $5,000 to Democratic Oregon Sen. Ron Wyden's reelection campaign in 2015, according to FEC filings.
Before he took on the diplomat role, Sondland's work in government had been limited. He was appointed by George W. Bush to serve on the Commission for White House Fellowships.
He had worked on the transition team for Oregon Democratic Gov. Ted Kulongoski, who was governor from 2003 to 2011. Sondland also worked as a principal Republican liaison for Oregon and the White House. He also chaired the governor's Office of Film and Television.
While he is ambassador to the European Union, he has stated that he has a specific interest in Ukraine.
"President Trump has not only honored me with the job of being the US ambassador to the EU, but he's also given me other special assignments, including Ukraine," he told a Ukraine media outlet in July.
Sondland replied to Taylor saying that the diplomat was "incorrect about President Trump's intentions."
"The President has been crystal clear no quid pro quo's of any kind. The President is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelensky promised during his campaign," Sondland wrote in his text to Taylor.
A source told CNN that Sondland had called the President after Taylor raised concerns and that Trump emphatically told him there had been no quid pro quo in his conversation with Zelensky.
Along with the subpoena, the chairmen informed Pompeo in a separate letter that they had scheduled depositions for five State Department officials, including Sondland, who have been mentioned in relation to the impeachment inquiry.
Last week, the State Department ordered Sondland not to appear before Congress.
"He is a sitting ambassador and employee of State and is required to follow their direction," Sondland's attorney Robert Luskin said at the time.
This story has been updated.
CNN's Jeremy Herb, Katelyn Polantz, Gloria Borger and Jennifer Hansler contributed to this report.