It also comes as Kane prepares to release a much-anticipated report she commissioned on how her predecessors handled the Jerry Sandusky child sexual abuse investigation. She said this week that the report could be made public very soon.
Kane's office on Tuesday revealed little about why King was leaving, except to say that his departure was expected.
In an interview, King said that he was "honored" to work for Kane, but that his job was "always intended to be a short-term assignment."
Said King: "Kathleen Kane has been a great friend of mine for a long time, and I was honored that she asked me to help get her administration up and running - and extremely pleased to work on issues that are important to the safety of Pennsylvanians and the success of the commonwealth."
Several people close to the Philadelphia lawyer, who plans to return to private practice, said he never intended to stay with Kane's office longer than 18 months.
In his 16 months on the job, King has had a hand in virtually every high-profile decision the office has made, from helping Kane formulate her position on why she would not defend the state's ban on same-sex marriage to helping Pennsylvania recoup about $120 million in contested tobacco lawsuit settlement money.
King, who worked in Ed Rendell's gubernatorial administration, also was instrumental in bringing in former federal prosecutor H. Geoffrey Moulton to investigate whether there were any delays in the office's handling of the Sandusky case. That investigation was carried out by Kane's three predecessors, among them Gov. Corbett, who was the attorney general until the beginning of 2011. Sandusky, a former top assistant to Pennsylvania State University football coach Joe Paterno, was not charged until late in 2011 and convicted the next year.
Philadelphia lawyer Mark Aronchick, one of the lawyers who brought the lawsuit that led to the reversal of Pennsylvania's gay marriage ban, called King "a perfect example of a devoted public servant who has that ability to move in and out of government seamlessly - and get the job done."
Drew Crompton, counsel and chief of staff to Senate President Pro Tempore Joe Scarnati (R., Jefferson), put it this way: "I always had the sense that he dealt with us fairly and squarely."
King's duties will be taken on by Bruce Beemer, senior counsel to Kane and chief deputy attorney general of the office's criminal prosecutions section.