Supporters of the Glasgow-born man say he acted through naivety as a result of Asperger's syndrome, a form of autism, and should not be considered a criminal. But US prosecutor Paul McNulty accused him of using the codename Solo to commit the biggest military hack of all time.
Mr McKinnon, 46, was born in Glasgow in February 1966. He and his family lived in Milngavie, East Dunbartonshire, before moving to north London when he was young. He was just 14 when he got an Atari computer, leading to an obsession with space invader video games. He later joined the British UFO Research Association where he found like-minded enthusiasts. When he learned his stepfather was from Bonnybridge, Stirlingshire, known as the UFO capital of Scotland, became even more fascinated by the subject.
Mr McKinnon's long-running case has attracted the backing of celebrities, politicians and other campaigners over the 10 years he has been trying every avenue open to him to avoid extradition. Pink Floyd musician David Gilmour sang on the Chicago protest song in support of Mr McKinnon, Sting said the case was a "travesty of human rights" while his wife, producer Trudie Styler, said that while Mr McKinnon's actions were "clearly misguided", jailing him in the US would be "an unnecessarily cruel and undignified way to treat anyone".
Actress Julie Christie added she hoped he "will not become yet another victim of the American criminal justice system because this country did not have the courage to stand up to bullying demands".
Lord Carlile, the former independent adviser on anti-terror laws, said extraditing the Scot would be "cruel and unconscionable" when he could be prosecuted in the UK. David Cameron raised the case twice with US President Barack Obama, and said that he was "far from convinced" extradition was the right course of action.
Mr McKinnon began to search for proof of UFOs after watching the movie WarGames in which a teenager brings the world to the brink of war by hacking into the Pentagon computers. From the bedroom of his girlfriend's aunt's house in Wood Green, north London, he hacked into 97 American military computers at the Pentagon and Nasa between 2001 and 2002.
He was caught in 2002 as he tried to download a grainy image he believed was an alien spacecraft on a Nasa computer housed in the Johnson Space Centre in Houston, Texas. But before the download was complete, Mr McKinnon was disconnected and soon the US authorities tracked him down via his email account.