After 26 years in the city prosecutor's office, First Assistant District Attorney Ed McCann resigned Monday, officials said.
George D. Mosee Jr., the head of the office's juvenile division, will replace him, District Attorney Seth Williams said in a statement.
McCann declined to say why he was leaving - although his name has been floated in courthouse circles as a possible addition to Mayor-elect Jim Kenney's administration.
"I'm really proud of my 26 years of service," McCann said. "It's been a privilege to work with a group of such dedicated, talented people."
He said the decision to leave, effective immediately, was his.
"We'll see what's next," McCann said. "I hope, in one way or the other, to continue to be involved with some of the things I'm really passionate about."
McCann started work at the District Attorney's Office directly after graduating from Temple Law School, and has taken the lead on some of the city's most high-profile cases in more than two decades on the job.
He worked on several cases involving police officers killed in the line of duty. In 2011, he won three convictions in the death of Danieal Kelly, a 14-year-old girl with cerebral palsy who was found starved and covered in bedsores in a bedroom in her mother's West Philadelphia apartment.
The case shocked the city and led to sweeping changes in the city's Department of Human Services.
"We made people pay attention to a 14-year-old girl who no one paid attention to during her life," McCann said.
Mosee has worked for the office since 1988 and has headed the juvenile division since 2002. He said in a news release that he was "humbled" by his selection for the position, and committed to Williams' "ongoing reforms and initiatives to make this office the very best it can be."
Cameron Kline, spokesman for the District Attorney's Office, said Mosee was not available for further comment.