Mr Assange, declared by the Donald Trump administration as US public enemy number one, was speaking ahead of a live Spanish television interview.
He told current affairs show When It's Gone: "The CIA is basically useless. They are extremely incompetent as an organisation.
"It is the organisation that gave us the end of democracy in Iran, Pinochet, the destruction of Libya, the rise of ISIS within Libya, al-Qaeda, the Syrian disaster and the Iraq war.
"It is one of the most useless organisations in the world."
US intelligence agencies have concluded that Russia was behind the hack, and used Wikileaks to harm the chances of Mrs Clinton and favour Mr Trump.
Mr Assange said the release was not intended to affect the election.
The US Justice Department has been investigating Mr Assange since his Wikileaks website posted thousands of embarrassing diplomatic cables stolen by a US Army soldier, now known as Chelsea Manning.
Mr Assange was granted asylum by Ecuador in June 2012 to prevent his extradition to Sweden, where he faces a sexual assault allegation, which he feared would then lead to his extradition to the US.
He remains confined to the Ecuadorian embassy in London, where police have said he will be arrested if he leaves the building.
He told When It's Gone "the Embassy is better than a prison" but "monotony" was his real fear.
The live interview goes out on Spanish TV on Thursday night.
Comment: Further reading: Trump passively gives go-ahead for AG Sessions to bring down Assange and Wikileaks, not involved in that decision