The 19-year-old defender announced yesterday on Twitter that he had signed for Hibs.
Mullen, right, tweeted: “Delighted to have signed a deal at Hibs. Hard work starts now and can’t wait to get started.”
After eight years with the Tynecastle club, the former Scotland under-17 international did not have his contract renewed by Hearts at the end of the season, even before they went into administration.
Glasgow-born Mullen’s season ended prematurely when he suffered a fractured bone in his foot playing, ironically, against Hibs in March. He made his debut for Hearts in a Scottish Cup tie against Auchinleck Talbot in January 2012, but made no further appearances until January this year, after which he played ten times for the club.
Moves across the derby divide are rare. Mullen will be the first player since Michael Stewart in 2005 to transfer directly from Hearts to Hibs, with Brian Hamilton moving from Hibs to Hearts in 1995.
A Hibs spokesman yesterday confirmed Mullen had signed a one-year contract.