The track, taken from the US version of the singer's forthcoming debut album 'Sticks & Stones', entered the US iTunes Top 10 last month and has stayed there ever since.
The song was released as a download two months ago, but Lloyd has been on a heavy radio tour of the US to promote the single and album.
Lloyd told Music Week: "It's so amazing Want U Back has sold one million copies in America. My Brats rock, thank you!"
Chairman and CEO of Epic Records, LA Reid, added: “I’m very excited about the progress of Cher’s success. This is only the beginning!”
'Sticks & Stones' is now set for release in the US on 2 October on Syco/Epic Records and will feature a slightly different tracklisting to the UK version.
The album will include new songs as well as guest spots from the likes of Snoop Dogg, who appears on the album version of 'Want U Back'.
Lloyd is also set to appear on US TV's the Today Show on 30 August, after her performance on America's Got Talent last month was praised by US celebrity blogger Perez Hilton.
The success and praise in America will help Lloyd to forget about her performance at V Festival in Chelmsford, Essex, last weekend, when she was pelted with cups of urine by some crowd members.
Below: Cher Lloyd at V - before it all went wrong