Appearing by video leak to give evidence on Mr Assange's behalf at an extradition hearing at the Old Bailey in London, Eric Lewis said if the 49-year-old was found guilty of the charges he faces he could be sentenced to up to 175 years.
United States prosecutors have indicted the 49-year-old Australian on 17 espionage charges, and one of computer misuse, over WikiLeaks' publication of secret U.S. military documents a decade ago.
He said that the U.S. government takes a "very harsh view in respect of national security defendants".
"We are looking at a sentence somewhere between 20 years, if everything goes brilliantly, to 175 years, which the government could easily ask for", he said.
Mr Lewis, a senior partner at DC law firm Lewis Baach Kaufmann Middlemiss in Washington DC, added: "All signs point to a very long sentence, measured in many decades".
Mr Assange was arrested on April 11 a year ago at the Ecuadorean Embassy in London, after United States prosecutors in the Eastern District of Virginia unsealed a criminal case against him, alleging he conspired with Ms Manning to commit computer intrusion.
Mr Assange is accused of conspiring with former U.S. army intelligence analyst Chelsea Manning to hack into a Pentagon computer and release hundreds of thousands of secret diplomatic cables and military files on the wars in Iraq and Afghanistan.
Manning was sentenced to 35 years before it was later reduced to seven years.
Mr Assange's legal team claim the prosecution under the Donald Trump administration has been politically motivated.
He has spent a year in prison after skipping bail eight years ago to avoid being sent to Sweden over a sexual abuse case.
He said the USA attorney general was being "much tougher on leakers, including Mr Assange". He has been in a British prison since he was ejected from his refuge at the Ecuadorian embassy in London in April 2019.