Khalid Ali, a plumber-turned-terrorist, was arrested in April 2017 in Parliament Street, where he was caught carrying three knives.
Ali had recently returned from Afghanistan, where he made and detonated bombs, the Old Bailey heard.
A jury convicted Ali of preparing an act of terrorism in the UK, and two counts of possessing an explosive substance with intent, the BBC reported.
He said his conviction demonstrates "how close the UK counter-terrorism network works with partners and allies to bring to justice those seeking to harm others across the world".
Prosecutor Brian Altman had told the jury that Ali planned a "deadly terror attack at the very heart of this country's democracy by killing a police officer, a member of the military or even a Parliamentarian".
He added that only the intervention of the police had prevented "another murderous terror attack in Westminster".
Ali travelled to Afghanistan in 2011. He did not return to the UK until November 2016, when he was stopped at Heathrow airport, interviewed by police and his fingerprints and DNA samples were taken.
Defending the decision not to arrest Ali until April 2017 - when he was armed and within metres of Parliament - Haydon said police and security services were "managing any potential risk he posed and he was arrested at the most appropriate time".
During questioning after his arrest, Ali admitted fighting British soldiers in Afghanistan, but refused to say whether he had killed any.
In a police interview shown during the trial, Ali also said he had detonated more than 300 bombs.
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