Jennifer Robinson said allegations of Assange's gross behaviour, including that he had smeared walls with faeces, were fabricated as a pretext to force him out.
The WikiLeaks founder was holed up in the embassy for more than seven years before he was arrested on Thursday.
Ecuador has claimed Assange had to be reminded to flush the toilet, left dirty underwear in the lavatory, did not clean dishes and left the cooker on.
Ms Robinson told Sky's Sophy Ridge: "The first thing to say is Ecuador has been making some outrageous allegations. It's a difficult situation.
"Ecuador has made these allegations to justify the unlawful and extraordinary act of letting police come inside an embassy.
"I've been visiting him for the last seven years. This man has been inside a room with no outside access. Inside the embassy it's become more difficult. The politics changed when Ecuador's political situation changed with a new leader.
"He stayed inside the embassy for so long because of a real and legitimate fear of US extradition which, as we saw on Thursday, proved to be justifiable fears."
Assange is facing charges in the US for "conspiracy to commit computer intrusion" in connection with documents published in 2010.
Ms Robinson welcomed support from Labour leader Jeremy Corbyn, who has said the UK should not extradite Assange to the US.
She said Assange had published the greatest amount of confidential US information in history and had been doing what journalists do every day.
"It is right and correct for Jeremy Corbyn to say what he said," said Ms Robinson.
The lawyer denied Assange had avoided allegations of rape and sexual misconduct in Sweden by remaining in the embassy.
Assange was accused of rape and other sexual offences, against two women, following a WikiLeaks conference in Stockholm in 2010 - allegations he has vehemently denied.
The rape allegation facing the 47-year-old has a limitation period which expires in August 2020.
Ms Robinson said: "This was and is not about avoiding facing Swedish justice. It is about avoiding US injustice. He has cooperated with the Swedish investigation. Swedish prosecutors came into the embassy to hear his testimony. After that they closed the case."
Swedish authorities said at the time they were setting the case aside because they had exhausted all avenues for as long as Assange stayed inside the embassy.
Ms Robinson said that if the investigation was reopened "we will deal with it".
Assange was removed from the embassy after the Ecuadorian government withdrew his asylum - accusing him of interfering in international affairs and being discourteous to embassy staff.
America has just under two months to present Britain with a detailed case to justify Assange's potential extradition, a US government official has said.
Assange appeared at Westminster Magistrates' Court on Thursday where he was found guilty of breaching his bail conditions, meaning he also faces a jail term of up to 12 months in the UK when he is sentenced later at crown court.
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