The 49-year-old and his co-defendant Mohammed Mizanur Rahman pledged allegiance to Isis during its brutal advance through Syria and Iraq in 2014 and used videos to support the terrorist group's cause, police said.
And security sources now believe Choudary has links to as many as 500 jihadis who have left Britain to join Isis in Syria and Iraq.
He and Rahman were found guilty on 28 July following a trial at the Old Bailey but the verdict was not announced until Tuesday because of reporting restrictions.
Choudary, who was previously a spokesperson for the banned Islam4UK group, denied the charges but was convicted over a series of lectures put on YouTube between June 2014 and March last year.
He and Rahman, of Palmers Green in London, were charged under section 12 of the Terrorism Act after being arrested on suspicion of being members of a proscribed organisation on 25 September 2014.
Speaking to Sky News before his conviction, Choudary claimed he was exercising his right to freedom of speech and had not broken the law.
“If you look at my speeches, I have said the same thing for 20 years. For me, it is a matter of worship,” he said.
“If people are implementing the Sharia, then I cannot shy away from what the divine text says in relationship to that.
“If you cannot say when you believe in something and you cannot share that view, then you don't really have freedom to express yourself in this country.”
Choudary, of Ilford, co-founded the Islamist group al-Muhajiroun, which was named by Scotland Yard as the “driving force” behind a number of people who later committed terrorist attacks including the 7/7 bombers and Drummer Lee Rigby's murderers.
Among al-Muhajiroun’s members was Siddhartha Dhar, known as Abu Rumaysah, an aide of Choudary’s who would later travel to Syria to fight for Isis, boasting of evading British security forces online and posting a photo of himself holding his baby son in one arm and a Kalashnikov in the other.
Dhar is suspected of being the masked executioner in a propaganda video released in January, apparently replacing the British militant Mohammed Emwazi after he was killed in a drone strike.
Choudary was also a prominent member of Islam4UK, al-Ghurabaa and The Saved Sect, which were proscribed as aliases for the same terrorist organisation by the British Government.
A spokesperson for the Metropolitan Police said Choudary and Rahman have been “recruiters and radicalisers for over 20 years", who were called upon to pledge their support to Isis and encourage Muslims in the UK to join its ranks when the group declared its "caliphate" 2014.
At a meeting in a restaurant on 2 July of that year, the men spoke to convicted Indonesian terrorist Mohammed Fachry by phone and pledged their allegiance to Isis and its leader Abu Bakr al-Baghdadi.
Fachry then published the oath on an Indonesian website, giving police the evidence investigators said was needed to show they "stepped over the line”.
On the same day, Choudary posted a series of tweets praising the "caliphate" and urging Muslims to travel to Dar ul-Islam (Isis territory) – part of a series of messages praising Isis.
Commander Dean Haydon, head of the Met’s counter-terror command, said Choudary and Rahman’s prosecution was “significant” in the UK's fight against terrorism.
“These men have stayed just within the law for many years, but there is no one within the counter terrorism world that has any doubts of the influence that they have had, the hate they have spread and the people that they have encouraged to join terrorist organisations,” he said.
"Over and over again we have seen people on trial for the most serious offences who have attended lectures or speeches given by these men.
"This has been a significant prosecution in our fight against terrorism and we will now be working with communities to ensure that they are not replaced by others spreading hate.”
Investigators analysed decades of evidence for the case, including 333 electronic devices containing 12.1 terabytes of data, and worked with authorities in Indonesia.
Choudary and Rahman are due to be sentenced at the Old Bailey on 6 September.