Lord Justice Stanley Burnton and Justice Wilkie dismissed claims by McKinnon's lawyers that legal moves to extradite him to the US would breach his rights under Article 3 of the Human Rights Act - which prohibits torture, inhuman and degrading treatment.
McKinnon would find extradition, trial and imprisonment in the US "very difficult indeed," the judgement concluded. "His mental health will suffer. There are risks of worse, including suicide. But... his case does not approach Article 3 severity."
McKinnon and his supporters said they were deeply frustrated and angry at the judgement. Karen Todner, McKinnon's solicitor, said it was an affront to British justice that someone as clearly vulnerable as McKinnon could be extradited.
Precedent held that Article 3 could be breached if the country where someone was extradited was likely to actively subject someone to torture, or inhumane or degrading treatment, the court said.
But it was harder to argue that conditions in another country were so poor that it would have so severe an effect on someone's ill health.
The US had given compelling assurances that McKinnon's medical reports would be considered not only by his jailors but also by the US courts. He would undergo further psychological assessment and would be given support comparable to that he would get in a British jail and justice system.
Evidence presented by the National Autistic Society that "suicide and self harm can be associated with the [psychological] disorder" demonstrated how uncertain were the risks to McKinnon's health, the court concluded.
Legal precedent would only allow the court to intervene if the risks to McKinnon's health were real and not merely more than "fanciful".
McKinnon sought a UK prosecution in the hope that it would prevent his extradition. But the court judged that the Director of Public prosecutions should be left to decide whether to prosecute McKinnon. The DPP had decided that a UK prosecution was not possible, so US prosecutors should be trusted to get on with it.
The court gave McKinnon leave to appeal its judgement before the House of Lords. There was room to consider further the question of mental health under Article 3 of the ECHR.
The courts said it would also consider giving leave to appeal under Article 8, which gives someone the right to a family life.
Janis Sharp, McKinnon's mother, condemned the law for judging it "fair to destroy someone's life in this way".