Lord Justice Maurice Kay said: "Mr McKinnon's conduct was intentional and calculated to influence and affect the US government by intimidation and coercion."
"As a result of his conduct, damage was caused to computers by impairing their integrity, availability and operation of programmes, systems, information and data on the computers, rendering them unreliable," Kay said.
McKinnon's lawyers argued that sending him to the US would be a breach of the 41-year-old's human rights.
Ben Cooper said he would now try to have the case heard in the House of Lords, Britain's highest court, including an alleged threat from US prosecutors that McKinnon "would fry."
McKinnon and his lawyers expressed fears he could be held indefinitely without trial at the US prison camp in Guantanamo Bay, Cuba.
But the US government said it had given assurances to the British government that it would not make McKinnon subject to "Military Order Number One", which allows the US president to detain suspects indefinitely.
McKinnon, whose hacking name was "Solo," has admitted gaining access to US government computers but denies causing any damage.
Paul McNulty, the US Attorney for the eastern district of Virginia, said "Mr McKinnon is charged with the biggest military computer hack of all time."
If found guilty in the US, McKinnon could face up to 70 years in jail and be fined up to $1.75 million.