The Harry Potter actress picked up the trophy for her portrayal of the U.K.'s late Northern Ireland Secretary Mo Mowlam in biopic Mo.
Former Doctor Who star Christopher Eccleston was named Best Actor for BBC1 drama Accused, which also picked up Best Drama Series at the glitzy event.
Other British productions to triumph at the New York ceremony included Gareth Malone Goes to Glyndebourne for best arts programme, while the production company which created the show, Twenty Twenty Television, won the non-scripted entertainment award for The World's Strictest Parents.
Pop superstar Lady Gaga was a surprise attendee at the ceremony, which was hosted by Jason Priestley at the Big Apple's Hilton Hotel, when she turned up on stage to present an honorary prize to American Idol's executive producer, Nigel Lythgoe.
The Poker Face hitmaker branded the Brit her favourite TV executive at the event.
Other winners included Canadian show Life with Murder, which picked up the best documentary prize, and Swedish crime drama Millennium, which was honoured with the best TV movie or mini-series title.
The International Emmys honour the best TV productions created outside of the U.S.