Assange, 52, will plead guilty to a single criminal count of conspiring to obtain and disclose classified US national defence documents and is expected to be sentenced to 62 months of time already served at a hearing in Saipan in the Northern Mariana Islands.
He will then be free to return home to Australia.
Stella Assange, a lawyer who has worked on his campaign almost since the start of his legal battles, said she was elated at the move but still angry that he had been held for so long.
The couple married while he was in Belmarsh maximum-security jail in London, and they had two children while he was holed up in the Ecuadorian embassy in London.
She said they would seek a pardon because the acceptance of guilt on an espionage charge was a "very serious concern" for journalists around the world.
She also said they would launch a fundraising campaign as the flight from London to Saipan for a court hearing and then to Australia would cost around half a million US dollars ($A750,000).
"The fact that there is a guilty plea, under the Espionage Act in relation to obtaining and disclosing national defence information is obviously a very serious concern for journalists and national security journalists in general," she said.
Stella said she had always believed this moment would come but she would not relax until he had arrived in Australia, where he is expected on Wednesday.
She said on Tuesday morning that she had still not broken the news to their children.
"I feel elated. I also feel worried, you know, because I'm so used to this. Anything could happen. I'm worried that until it's fully signed off, I worry, but it looks like we've got there," she said, speaking from Sydney.
Stella said Julian had been a "so much lighter" as the agreement became closer and they would likely spend time in Australia now - a good place, she said, to regain your health and sanity.
"I think that goes not just for Julian but for me as well," she said.
"It's been a rough, very rough few years and we need some time."
First, she said they needed to raise money.
"It's Australian policy that he will have to pay his own return flight so he's had to charter a flight and so he will basically be in debt when he lands in Canberra.
"We're going to launch an emergency fund to try to get this money so that we can pay the Australian government back for his freedom flight.
"It's half a million US dollars."
