Conley's term ends in January and he did not seek re-election. He announced Wednesday he will be going to work for Mintz Levin and the firm's lobbying arm, ML Strategies.
He plans to step down on Sept. 26 and expects that Republican Gov. Charlie Baker will appoint an interim district attorney to fill out his term.
Rachael Rollins won a 5-way Democratic primary on Sept. 4. Rollins, a former attorney for state transportation agencies, faces independent candidate Mike Maloney in November.
Conley has been the top prosecutor since 2002 in the county that includes Boston, Chelsea, Revere and Winthrop. He ran unsuccessfully for mayor of Boston in 2013.
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