The 51-year-old pilot, Erwin Vermont Washington of Lakewood, Colorado, was breath-tested and taken to London's Heathrow police station.
The plane's departure to Chicago was "imminent", with 124 passengers and 11 crew members on board, when the arrest took place, a Heathrow spokesman said.
Passengers were put on to other planes after the flight was cancelled.
A United Airlines spokeswoman said: "Safety is our highest priority and the pilot has been removed from service while we are cooperating with authorities and conducting a full investigation.
The pilot was reported to police by United Airlines staff who suspected him of being drunk.
Link here and here.