Suffolk District Attorney Dan Conley will be resigning effective Sept. 26 to become senior counsel for Mintz Levin and senior advisor for ML Strategies, the Suffolk County District Attorney announced Wednesday from his Boston offices.

He has held the post since 2002, and Gov. Charlie Baker will appoint an interim DA for the remainder of Conley's term.

"As I told many of you when I announced that I wouldn’t seek a fifth term, there are still professional opportunities and challenges that I’d like to pursue in my legal career," Conley said. "So I was honored when Bob Popeo, Chairman of Mintz, Levin, Cohn, Ferris & Popeo, and Steve Tocco, President & CEO of ML Strategies, offered me the tremendous opportunity to join their prestigious organization."

Conley said he hopes to draw on his quarter century as City Councillor and district attorney in the new post, which will bring him "back to the hands-on practice of law that first inspired me."

In his last expected press conference in the role, Conley thanked the members of his office and reflected on the changing county over his tenure.

"Boston and Suffolk County bear almost no resemblance to what they were when I was first sworn in as an Assistant District Attorney in 1984," he said, according to prepared remarks. "In fact, they’re dramatically different even from when I took office as DA in 2002. They’re growing and thriving because they’re safer, and they’re safer because our criminal justice system is more accurate, more effective, and more fair than ever before."

He noted the decline in violent crime in Boston, which fell by almost 35 percent, and simultaneous reductions in committed sentences at the Suffolk County House of Correction by 40 percent. Juvenile prosecutions were cut almost by half.

Conley announced in February that he would not run for re-reection after 16 years, kicking off a competitive race to replace him. Rachael Rollins, the former chief legal counsel of the MBTA and MassDOT, won the Democratic primary last week, beating out Conley's endorsed pick.

He told reporters that he knows Rollins well, having met her many times and both texted and spoke with her after the primary. Conley will support her "100 percent" he said.

In an interview Tuesday, before the announcement that Conely would leave early, Rollins praised the office's diversionary program and the departing district attorney.

"I have his cell phone number," she joked. "I'm going to be bothering Dan Conley; whether he wants to pick up or not is up to him. But I have respect for him. I know he cares about victims. I know he cares about keeping the community safe, and I know this isn't an easy job."

The general election is set for Nov. 6, where Rollins will face independent candidate and Brockton attorney Mike Maloney. Until Conley's long-tern successor is elected, Baker's appointee must be a member of the bar in good standing and a resident of Suffolk County.

This is a developing story.

Further reading: After primary victory, Rollins hopes to give DA office ‘another voice, another perspective’