The whip and MP for Bristol East will act as spokesperson for the Labour Party's activities in new media, and will aim to improve the way MPs and ministers are engaging online.
She was appointed by international development secretary Douglas Alexander in his role as general election coordinator.
The Independent named McCarthy as the most influential MP on Twitter last month, beating former minister for digital engagement Tom Watson to the top spot.
McCarthy said that she is hoping to talk to MPs and PCCs about the new media campaigning they can do at a local level.
The Labour new media chief had hoped to hold the news until today but the story was inadvertantly leaked by Guardian.co.uk a day early.