The home secretary also denied extradition law was wrong, arguing it was appropriate for "an age where crime is increasingly indifferent to national borders".
Glasgow-born Mr McKinnon could face 60 years or more in prison if convicted in the US.
He admits hacking by accessing 97 government computers belonging to organisations such as the US Navy and Nasa, but denies it was malicious. He also denies the allegation he caused damage costing $800,000 (£487,000).
Mr McKinnon has always insisted he was looking for classified documents on UFOs, which he believed the US authorities had suppressed.
He has challenged refusals by the home secretary and the director of public prosecutions (DPP) to try him in the UK.
US-UK EXTRADITION TREATY
2003 treaty, agreed in aftermath of 9/11 attacks
Offence must be punishable by one year or more in jail in both countries
US has to prove "reasonable suspicion" for extradition of a British citizen
To extradite an American from the US, British must prove "probable cause"
Since 2004, 46 people have been sent from the UK to the US for trial, and 27 from the US to the UK
But the DPP refused to order a UK trial, saying the bulk of the evidence was located in the US and Mr McKinnon's actions were directed against the US military infrastructure.She also said it was not the place of ministers to intervene in the justice system.
And two judges rejected his court bid to avoid extradition, ruling that it was "a lawful and proportionate response" to his offence, even though they conceded he might find extradition and prison in the US "very difficult indeed".
Mr McKinnon has already appealed unsuccessfully to the House of Lords and the European Court of Human Rights.
But the case has led to a political row, with Tory leader David Cameron saying it raised "serious questions" about the extradition pact between the US and UK.
Liberal Democrat home affairs spokesman Chris Huhne has argued the American government would not "hang one of their citizens out to dry in the same way".
A letter has been sent to US President Obama signed by 40 British MPs asking him to step in and "bring this shameful episode to an end". Ms Sharp has also called on the president to intervene.
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