The British-born 49-year-old backed the terrorist group in a series of talks posted on YouTube, and recognised a caliphate - a symbolic Islamic state - had been created under an IS leader after it was announced on 29 June 2014.
Choudary, who was sentenced alongside Mohammed Mizanur Rahman, was jailed for five years and six months. Rahman received the same sentence.
Supporters in the public gallery at The Old Bailey shouted “Allahu Akbar” - “God is greatest” - as Choudary was sentenced.
Mr Justice Holroyde told the court that a “significant proportion of those listening” to Choudary’s words “would be impressionable persons looking to you for guidance on how to act”.
Choudary’s lawyer, Mark Summers QC, earlier told the court that his client expected to spend a substantial portion “if not all” of his sentence in solitary confinement.
He said Choudary, “on reflection”, would have done things differently, “had he known the boundaries of the law”.
Choudary was now, Summers said, “determined not to cross those boundaries in the future”.
Jo Sidhu QC, who represented Choudary’s co-accused, Mohammed Mizanur Rahman, said there was no evidence his client had inspired anyone to join IS.
Sidhu said the risk of radicialising other was “not substantial” as the conversations taking place were between people of “similar minds”.
He added that his client never sought to attack Britain - he lived within the rules and observed the “Covenant of Security”.
Despite being a leading figure in the banned group al-Muhajiroun (ALM), and with a series of former supporters going on to be convicted of terrorism, Choudary stayed on the right side of the law for two decades.
But the pledge of allegiance posted online provided a turning point for police who swooped to arrest Choudary and Rahman, 33, from Whitechapel, east London.
Following an Old Bailey trial, the pair were found guilty of inviting support for IS between 29 June 2014 and 6 March 2015 and remanded in custody.
For legal reasons, details of the case could not be reported until three weeks after the guilty verdicts were delivered on July 28.
Choudary faced a maximum possible sentence of 10 years in prison, although Justice Holroyde said there was little precedent for such cases.
The trial heard that the preacher, viewed by officers as a key force in radicalising young Muslims, had been the “mouthpiece” of Omar Bakri Mohammed - the founder of the banned extremist group ALM.
He courted publicity by voicing controversial views on Sharia law, while building up a following of thousands through social media, demonstrations and lectures around the world.
In one speech in March 2013, Choudary, from Ilford, north-east London, set out his ambitions for the Muslim faith to “dominate the whole world”.
He said: “Next time when your child is at school and the teacher says, ‘What do you want when you grow up? What is your ambition?’, they should say, ‘To dominate the whole world by Islam, including Britain - that is my ambition’.”
Supporters included Michael Adebolajo and Michael Adebowale, the murderers of Fusilier Lee Rigby, and suspected IS executioner Siddhartha Dhar.
Shortly after the announcement of the caliphate, Choudary held a meeting with his closest aides at a curry house in Mile End Road in east London to discuss it.
Before accepting it was legitimate, he also consulted his “spiritual guide”, Omar Bakri Mohammed, currently in jail in Lebanon, and Mohammed Fachry, the head of ALM in Indonesia.
On July 7 2014, the trio’s names appeared alongside Rahman’s on the oath posted on the internet, which stated the Muhajiroun had “affirmed” the legitimacy of the “proclaimed Islamic Caliphate State”.
The defendants followed up by posting on YouTube a series of lectures on the caliphate, which Choudary promoted to more than 32,000 Twitter followers.
The married father-of-five denied encouraging his followers to back the terror group and insisted the oath had been made without his knowledge. He said of the pledge: “It is completely unnecessary. For the rest of the Muslims it is obedience from the heart.”
Despite protesting his innocence, he continued to express extreme views, refusing to denounce the execution of journalist James Foley by so-called Jihadi John, aka Mohammed Emwazi, in Syria in 2014.
He told the jury: “If you took an objective view, there are circumstances where someone could be punished.”
Following the convictions, Commander Dean Haydon, head of Scotland Yard’s counter-terrorism command, said: “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.
“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.
“The oath of allegiance was a turning point for the police - at last we had the evidence that they had stepped over the line and we could prove they supported Isis.”
Speaking outside court, Haydon said it was “a worry” that Choudary might wield influence over others in prison.
He would not put a figure on the number of people Choudary is believed to have influenced but added around 850 British people have travelled to IS in Syria.
Choudary’s conviction was also welcomed by leading British Muslims, who condemned his “evil” and “hateful” views.
Director of Public Prosecutions Alison Saunders told ITV News: “Anjem Choudary has been very careful to walk a line where he didn’t cross over unto an offence - until recently.
“Prosecutors have been looking and working with the police for a long time to make sure that we were able to bring him to justice.”
After Choudary’s conviction, Twitter removed Choudary’s Twitter account was removed by the social media giant.