Metropolitan police officers were called to the plane at Terminal 2 after the United Airlines captain claimed the marshal "stank of booze".
A police spokesman said the man was breathalysed and arrested last Monday, but released from custody a short time later with no further action.
The marshal - carrying a concealed loaded weapon - was taken to the airport's nick and breathalysed. He was reportedly one of the four air marshals on board the transatlantic flight.
A source on the Boeing 777 said yesterday after up to 266 passengers aboard Flight 935 were left fuming: "The captain smelt alcohol on the breath of one of four armed US air marshals who were on board.
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"It caused quite a scene," The Sun quoted the source as saying.
The federal marshal - whose duty in the wake of 9/11 is to "detect, deter and defeat hostile acts" - was later freed without charge because he had not broken any UK laws.
A police source said: "It is only an offence if someone has an aviation function like flying the plane or being an engineer.