A city employee was busted Tuesday night for dealing heroin in front of Manhattan​’s​ Municipal Building​,​ where he works as an exam monitor, according to a law-enforcement source.

Benjamin Henderson, who works for the Department of Citywide Administrative Services, allegedly sold over $8,000 worth of heroin and cocaine to undercover​ officers on ​seven occasions, the source said.

Four of the sales occurred near the entrance of the Brooklyn Bridge​-City Hall subway station directly beneath the historic building at 1 Centre Street, authorities said.

During one sale, Henderson allegedly flashed an undercover his DCAS badge.

And the shameless city employee even peddled cocaine near the New York-French American Charter School in Harlem, a law-enforcement source said.

He’s expected to be arraigned Wednesday afternoon in Manhattan Criminal Court.