Rupert Murdoch's News Corporation today announced the change at the top of News International - owner of papers such as The Sun and The Times.
He also revealed details of the planned separation of its $60 billion News Corp business into individual arms, which will split off the valuable TV and film assets, from the struggling and scandal-hit publishing businesses.
Darcey has been Sky COO since 2006, and in that time has contributed to the massive growth of the pay-TV giant, which is 39% owned by News Corporation.
He will step into the chief executive role at News International in the New Year, tasked with turning around the publisher that is still dealing with the fallout of the phone hacking affair from last year.
Rupert Murdoch, the chairman and chief executive of News Corp, said that he was "delighted" Darcey was going to take over at News International.
"Mike is a world-class executive with unprecedented strategic and commercial experience and I look forward to benefiting from his many talents," he said.
"His broadcasting background will provide important leadership in the development of our already impressive suite of digital products at News International."
Mockridge, previously chief executive of Sky Italia, was parachuted in as boss of News International in July 2011, when Rebekah Brooks resigned following the hacking scandal at the News of the World.
He has opted to leave the role and also News Corp itself after 22 years of service, amid reports that he was passed over for the most senior executive positions in the planned News Corp restructure.
As announced earlier in the year, News Corp will split into two separate businesses; one will include all the highly profitable film and television assets, such as the Fox network and Twentieth Century Fox movie studio; and the other will contain the declining publishing businesses, such as the newspapers and book publisher Harper Collins.
The media and entertainment arm will be renamed 'Fox Group' and have Rupert Murdoch as its chairman and chief executive, with current News Corp chief operating officer Chase Carey as its president and COO.
James Murdoch, who took a battering over his perceived lack of vigilance in the UK hacking scandal, will continue to serve as deputy chief operating officer to Carey.
The publishing arm will be headed up by Robert Thompson, who has served as editor-in-chief of Dow Jones and managing editor of The Wall Street Journal since 2008. He will start in the role as chief executive of the publishing group on January 1, 2013.
"This is an incredibly exciting time, for me personally, and for our companies' ambitious futures," said Rupert Murdoch.
"The challenges we face in the publishing and media industries are great, but the opportunities are greater."
He added: "Under Robert's leadership at News Corporation, we will build on our traditional mission to inform, entertain and enhance the lives of readers and viewers around the world, and relentlessly drive global growth by promoting excellence and investing in our businesses.
"At Fox Group, what began with the acquisition of a modest film studio over 25 years ago has grown into one of the world's most successful media companies of all times, defying conventional wisdom at every turn by pursuing excellence in creativity and innovation.
"Fox Group is perfectly positioned to deliver even more inspiring stories that engage audiences through film, television, sports and digital platforms, driving not only financial results but a lasting imprint on the millions of people who enjoy our various services, in every corner of the world."
Thompson said: "The global value of content is growing as the means of digital distribution multiply, providing us with a remarkable opportunity to profit from our employees' ingenuity and integrity.
"We have a collection of extremely successful companies, and our goal is to make the new News Corp far more than the sum of its distinguished parts."
Gerard Baker, currently deputy editor-in-chief of The Wall Street Journal, will succeed Thomson as editor-in-chief of Dow Jones and managing editor of The Wall Street Journal, effective from January 1, 2013.