President Bill Clinton was among those who paid tribute to Thomas on Tuesday.
Helen Thomas, who has covered White House affairs since the 1960s, did not say whether she was upset by the sale of UPI to News World Communications - also the owner of the Washington Times - but reports said she had strong objections.
UPI was sold in 1982 by the Scripps family. Before being sold on Monday, it went through two bankruptcy reorganisations and four owners, the latest being the Saudi operation, Worldwide News.
Thomas, 79, said, "I wish the new owners the best, great stories and happy landings."
Empty chair
The journalist has been a fixture at the White House since the presidency of John F Kennedy.
She is traditionally granted the first question at every White House presidential news conference.
President Bill Clinton was among those who paid tribute to Thomas on Tuesday.
"Presidents come and go, but Helen's been here for 40 years now, covering eight presidents and doubtless showing the ropes to countless young reporters and, I might add, more than a few press secretaries.
"Whatever she decides to do, I'll feel a little better about my country if I know she'll still be spending some time around here at the White House.
"After all, without her saying, 'Thank you, Mr President,' at least some of us might never have ended our news conferences," he said.
Friends said she planned to give speeches, write a book and try to syndicate her weekly column.
Spoof
Thomas was hired by UPI in 1943 as a writer for the local radio wire, earning $24 a week. In 1956, she was promoted to covering the Justice Department.
In 1960, Thomas was sent to cover John F Kennedy's post-election family holiday in Palm Beach, and she remained on the White House beat afterwards.
Thomas was named UPI's chief White House correspondent in 1970.
A few weeks ago, she made a cameo appearance in Bill Clinton's video spoof of his days in the White House.
Thomas played the lone reporter in a White House briefing room, dozing in the front row as the president made a statement.
When Mr Clinton called for questions, Thomas snapped awake, quipping: "Are you still here?"