Prosecutors said the Improvized Explosive Device (IED) contained ball bearings which would have acted as shrapnel causing devastating injuries, similar to that which occurred in the Manchester Arena attack May 22.
Smith, 20, made the device at home, put it in a black rucksack and boarded a Jubilee Line train. He got off at London Bridge and left the bomb on the train.
It was discovered at North Greenwich and made safe by police. Smith was later identified, arrested and charged with possession of an explosive substance with intent.
Smith was convicted after a trial at the Old Bailey in central London and on Friday, May 26, he appeared for sentencing. The judge was told by his defense lawyer "he had learned his lesson" and had vowed never to make another bomb.
The jury were shown a police interview with Smith in which he can be seen grinning throughout, apparently unaware of the seriousness of his actions.
UPDATED: Man who put explosives on tube found guilty https://t.co/BdJWCnPmmy— Chris Heflin-Scott (@CHeflinScott) May 3, 2017
Smith is Caucasian and is not believed to have had any connections with any Islamist group, although he built the bomb using an online al-Qaeda manual.
Judge Richard Marks QC told him: "I am influenced by your history of preoccupation with weapons and bombs as well as by your condition which makes it difficult for you… to understand and fully appreciate the very serious potential consequences of your actions."
Smith built the bomb using a US$3 clock from Tesco and followed directions in an al-Qaeda manual online.
His mother Antonitza Smith, 47, from Bermondsey, south London, had said previously:
"My son is not a terrorist. He needs help not prison."
She claimed he had watched a video about how to make a bomb on YouTube.
Mum of Devon student Damon Smith appeals for lenient sentence today after he was convicted of planting home made bomb on a London tube train pic.twitter.com/LHSXqwCQoq— Hamish Marshall (@bbchamish) May 26, 2017
"I think YouTube is poison personally. It should be banned. Because people with learning disabilities, if they get these funny ideas, they start copying.
"How to make a bomb, that shouldn't be on YouTube," Mrs. Smith said.
Commander Dean Haydon, head of the Metropolitan Police's Counter-Terrorism Command, said: "We have seen this week the horrifying impact a bomb can have. And whilst there is no connection between Manchester and Greenwich it brings into stark reality just how devastating it could have been. The bomb Smith made was a viable device, but it failed to detonate, which was our good fortune."