The Briton stands accused of hacking into top-secret US military computers, and faces a possible life-long prison sentence should his extradition go ahead.
McKinnon told BBC Radio 5 Live that he was "pretty broken up" by the ruling, but claimed that he had acted in the public interest.
"I am extremely sorry I did it, but I think the reaction is completely overstated," he said. "It felt like a moral crusade."
Glasgow-born McKinnon achieved worldwide fame after gaining access to 97 US military and Nasa computers in the biggest military hack in history.
McKinnon hacked into and disrupted numerous US military computers in 2001 and 2002 from his bedroom in North London. Since his arrest in 2002 he has never been formally charged in the UK.
McKinnon has consistently claimed that he is "a bumbling hacker" who was never a threat to security, and that he was only looking for UFO files that he believed the US government was keeping under wraps.
US prosecutors have offered McKinnon, aka 'Solo', a plea bargain. If he pleads guilty to two of the charges he will receive a four-year sentence. Otherwise, the 42 year-old faces a possible 70 years if found guilty of all the charges.
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