The drill involved participants including the Bank of England, the Treasury, the Cabinet Office and the Financial Conduct Authority, the chancellor said today.
The exercise was designed to test how good the two governments are at information sharing, incident response handling and public communications.
The two governments insisted the drill did not amount to a "cyber war game" or include "live play" - instead, it was designed to "enhance processes and mechanisms for maintaining shared awareness of cyber security threats between US and UK governments and the private sector".
George Osborne said: "Our national security is a priority for this government and that includes defending our cyber space.
"We train and prepare for the threat of a financial cyber-incident. That is why this exercise is so important, and we will continue to work with our partners in the US to enhance our cyber cooperation."
US Treasury Secretary Jacob J. Lew added:
"In our increasingly interconnected world, cyber criminals do not respect national borders and they target government, private industry and individual citizens alike.
"Confronting the cyber threat is a team effort that requires coordination at all levels. Today’s exercise with our U.K. partners is an important step to ensure that we are doing all we can to share threat information, adopt best practices and support our collective resiliency."