The Department of Investigation, New York City's anti-corruption watchdog, arrested more people in the last 12 months than in any year in the agency's 137-year history.

The 822 arrests in the fiscal year that ended Wednesday is a 12% increase from the prior one. The investigations have run the gamut, probing officials at the highest levels—resulting in the indictment of a sitting councilman—to a steady stream of charges against low-level employees for a variety of schemes.

In...