Louis B Susman claimed the legal agreement “continues to be widely and wrongly condemned by some in Parliament” and insisted the US had never denied a British extradition request.
The attack comes ahead of a debate in Parliament next week secured by MPs who believe the current arrangements are biased against British citizens.
They have raised the case of Gary McKinnon, who is facing extradition on computer-hacking allegations.
The US ambassador to the UK said the findings of an independent review last month by former Court of Appeal judge Sir Scott Baker ruled that the arrangements with the US were balanced.
In a statement to the Commons Foreign Affairs Committee he added: “Even so, the myths and inaccuracies persist. Many were repeated as recently as last week during a parliamentary debate and its subsequent press coverage.
“Accusations from Members of Parliament – such as ’24 Britons have been extradited to the US under the new arrangements and just one American to Britain’, or claims in the media that the Baker Review’s conclusions came ‘despite mountains of evidence to the contrary’ – are simply not true.”
Mr Susman insisted the treaty was “fair and balanced” and said the US government “strongly supports” it.
He added: “I believe that having signed the treaty, and having had it tested both through the British justice system and by independent experts, it is now incumbent on the UK Government to stand in support of it.
“This is what strong, enduring, bilateral alliances are built on – treaties and agreements that enshrine shared values and give us the legal authority to pursue common goals.”