Mr. Assange is expected to land in the Northern Mariana Islands, a US territory in the Pacific. On Tuesday, he left a British prison and stopped in Bangkok for refuelling on his way there.
US officials were after Mr. Assange, who is 52, for releasing a large batch of secret files in 2010, which they claimed endangered lives.
In response to the deal, which involves Mr. Assange pleading guilty to one charge, his wife Stella told the BBC she was "very happy".
During his long legal battle, Mr. Assange has spent the past five years in UK prisons, fighting against US efforts to extradite him.
He was also accused of rape and sexual assault in Sweden, charges he denied. To avoid extradition to the US, he spent seven years hiding in Ecuador's embassy in London.
Sweden dropped the case in 2019, citing the passage of too much time since the complaint. However, UK authorities later arrested him for failing to surrender to the courts for extradition to Sweden.
In the US, Mr. Assange was charged with conspiring to obtain and release national defense information after the major Wikileaks release in 2010.
Wikileaks published a video from a US military helicopter showing civilians being killed in Baghdad, Iraq.
It also leaked thousands of confidential documents suggesting that the US military had caused hundreds of civilian deaths in unreported incidents during the Afghanistan war.
The revelations became a major story, sparking global reactions and triggering intense scrutiny of America's role in foreign conflicts.
The Northern Mariana Islands, where Mr. Assange is expected to finalize his plea deal, are a remote group of islands in the Pacific and a US commonwealth. They are much closer to Australia than US federal courts in Hawaii or the mainland US.
In return for pleading guilty to one charge under the Espionage Act, Mr. Assange won't serve any time in US custody and will get credit for his time in UK jail.
According to a letter from the US Justice Department, he will eventually go back to Australia.
According to a post on X, Wikileaks stated that Mr. Assange left London's Belmarsh prison on Monday after spending 1,901 days in a small cell.
A video posted on the website showed Mr. Assange, wearing jeans and a blue shirt, being driven to London's Stansted Airport and later pictured inside the plane.
His brother Gabriel commented on the footage, saying it was nearly time for Mr. Assange's supporters to "have a drink and celebrate".
Stella Assange thanked Mr. Assange's supporters for their backing and expressed her joy that an agreement had been reached in an interview with BBC Radio 4.
She described the past few days as "touch-and-go" and "non-stop", and expressed feeling a whirlwind of emotions.
Mrs. Assange said she couldn't say much about the deal before her husband's court appearance, stating, "I don’t want to risk anything," during the Today programme.
She said her husband's priority was to "get healthy again," reconnect with nature, and for the family to have "time and privacy."
Stella also mentioned that their two young children were in Australia with her. She explained that she hadn't informed them yet about their father's imminent release, just that they were going to visit family and there was "a big surprise" in store for them.
Later on the BBC's Newshour programme, she said, "We haven't had much time to discuss the future. First up, he'll need to repay the Australian government $500,000 (£393,715) for the chartered flights."
Photos of the jet Mr. Assange used showed that its tail number matched an aircraft previously used by Taylor Swift in February.
Mr. Assange's lawyer, Richard Miller, refused to comment on the latest development when approached by CBS. The BBC has also reached out to his lawyer based in the US.
Mr. Assange's legal team and wife have argued for a long time that the case against him was politically driven, and they have urged US President Joe Biden to drop the charges.
In April, Mr. Biden mentioned he was reviewing a request from Australia, whose prime minister stated the case had "gone on for too long."
Mrs. Assange suggested a "breakthrough" occurred when the UK High Court started to review her husband's constitutional protections related to freedom of the press.
The reaction has been mixed. Former Vice-President Mike Pence strongly criticized the plea deal, calling it a "miscarriage of justice." On the other hand, the United Nations welcomed the news, with a spokesperson saying it marked significant progress toward resolving the case definitively.
Initially, US prosecutors aimed to try the Wikileaks founder on 18 charges, primarily under the Espionage Act. These charges stemmed from the release of confidential US military records and diplomatic messages concerning the wars in Afghanistan and Iraq, which they claimed put lives at risk.
Wikileaks, founded by Mr. Assange in 2006, claims to have published more than 10 million documents.
One of Mr. Assange's prominent collaborators, US Army intelligence analyst Chelsea Manning, was initially sentenced to 35 years in prison. However, President Barack Obama commuted her sentence in 2017.
During his lengthy legal battles, Mr. Assange has seldom been seen in public and has reportedly faced health issues, including a minor stroke in prison in 2021.
SOURCE: BBC