STORY: "We're going to fight this. We're going to use every appeal avenue," she told reporters.
"I'm going to spend every waking hour fighting for Julian until he is free, until justice is served."
British interior minister Priti Patel on Friday approved the extradition of Assange to the United States to face criminal charges, bringing his long-running legal saga closer to a conclusion.
Assange is wanted by U.S. authorities on 18 counts, including a spying charge, relating to WikiLeaks' release of vast troves of confidential U.S. military records and diplomatic cables which Washington said had put lives in danger.
His supporters say he is an anti-establishment hero who has been victimised because he exposed U.S. wrongdoing in conflicts in Afghanistan and Iraq, and that his prosecution is a politically motivated assault on journalism and free speech.
Originally, a British judge ruled that Assange should not be deported, saying his mental health meant he would be at risk of suicide if convicted and held in a maximum security prison.
But this was overturned on an appeal after the United States gave a package of assurances, including a pledge he could be transferred to Australia to serve any sentence.
Patel's decision does not mean the end of Australian-born Assange's legal fight which has been going on for more than a decade and could continue for many more months.
Assange's lawyer Jennifer Robinson told Reuters his case would be taken to the European Court of Human Rights if the appeal was not successful in the UK.