The English Defence League founder, whose real name is Stephen Yaxley-Lennon, was sued by Jamal Hijazi, who was assaulted in the playground at Almondbury Community School in Huddersfield in October 2018. Shortly after the video of the incident went viral, Mr Robinson claimed in two Facebook videos that Jamal was "not innocent and he violently attacks young English girls in his school".
In the clips viewed by nearly one million people, the 38-year-old also claimed Jamal "beat a girl black and blue" and "threatened to stab" another boy at his school, allegations the teenager denies.
At a four-day trial in April, Jamal's lawyers said that Mr Robinson's comments had "a devastating effect" on the schoolboy and his family who had come to the UK as refugees from Homs, Syria.
Mr Robinson, who represented himself, argued his comments were substantially true, claiming to have "uncovered dozens of accounts of aggressive, abusive and deceitful behaviour" by Jamal.
However, in a judgment delivered on Thursday, Mr Justice Nicklin ruled in Jamal's favour and granted him ￡100,000 in damages.
Catrin Evans QC, for Jamal, previously said that Robinson's comments led to the teenager "facing death threats and extremist agitation" and that he should receive damages of between ￡150,000 and ￡190,000.
During the trial, Ms Evans described Mr Robinson as "a well-known extreme-right advocate" with an "anti-Muslim agenda" who used social media to spread his views.
She added that Mr Robinson's videos "turned Jamal into the aggressor and the bully into a righteous white knight".
However, Mr Robinson maintained he was an independent journalist during the trial, telling the court: "The media simply had zero interest in the other side of this story, the uncomfortable truth."
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A hearing will follow Thursday's judgment to consider the consequences of the ruling.
In his judgment, Mr Justice Nicklin said Jamal Hijazi suffered "particularly severe" consequences due to Tommy Robinson's videos.
He said: "The defendant's allegations against the claimant were very serious and were published widely. The defendant has admitted that their publication has caused serious harm to the claimant's reputation."
Discussing the media attention that was on the original viral video, he added: "The defendant's contribution to this media frenzy was a deliberate effort to portray the claimant as being, far from an innocent victim, but in fact a violent aggressor.
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"Worse, the language used in the first and second videos was calculated to inflame the situation.
"As was entirely predictable, the claimant then became the target of abuse which ultimately led to him and his family having to leave their home, and the claimant to have to abandon his education.
"The defendant is responsible for this harm, some of the scars of which, particularly the impact on the claimant's education, are likely last for many years, if not a lifetime."
Mr Justice Nicklin rejected the allegations made as part of Mr Robinson's defence that Jamal had been abusive towards women.
He said: "I am satisfied that if the claimant had behaved so repeatedly in the abusive manner alleged, including to members of staff, then this would have been recorded in the claimant's school records.
"There is no trace of any such behaviour by the claimant in these records. On the contrary, his behavioural record is overwhelmingly positive."
The judge added: "In summary, the defendant has failed to demonstrate that the claimant had any propensity to behave in an aggressive or abusive manner towards girls and women."
More to follow...