Haider Ahmed, then a 17-year-old sixth form student, watched beheading videos at his parents’ home in Redhill, Surrey before going to buy a hunting knife.
The court heard he had become radicalised by violent gang ‘drill’ videos he watched on YouTube at the age of 14, before graduating to ISIS violence.
Ahmed, now 19, adopted an online persona in which he talked like a gangster but really spoke with a middle-class accent and refused to swear.
His online obsession led to him taking over an encrypted channel on Telegram dedicated to ISIS at the age of 16 and failing most of his GCSEs.
During the trial, jurors heard he bought a knife for £25 using Paypal, but his mother took it away after finding it in his bag.
Andrew Hall QC, who defended Ahmed, said: “He came across highly effective radicalisers who were fishing for the young, the isolated and the gullible.
“They found this boy in his bedroom in Surrey and he was brainwashed with a pernicious ideology and they turned him into their creature.”
Sentencing, Judge Peter Lodder QC said: “I very much hope that you take advantage of the programmes available to you in prison and realise that radicalisation is not the way forward for you.”
Outside court, Ahmed’s sister, Fizza Ahmed, said the government should take action on internet sites promoting IS ideology and she believed her brother was a victim.
“My brother was only 16, even less than that, when he began searching all of this and it shouldn’t have been accessible to him in the first place.
“I’m very disappointed there was not enough protection.”
Ahmed, of Wordsworth Mead, also pleaded guilty to four counts of disseminating violent IS propaganda, collecting a record of terrorist information and assisting another person to prepare acts of terrorism.
Det Ch Supt Kath Barnes, head of Counter Terrorism Policing South East, said: Ahmed was “a dangerous young man”.
“A prominent presence online, he possessed a significant amount of shocking material promoting a warped ideology.”