WASHINGTON—A top FBI official who helped oversee two politically sensitive investigations related to the 2016 presidential campaign is retiring from government service.

Bill Priestap, who currently serves as assistant director of the Federal Bureau of Investigation’s counterintelligence division, will leave his post by the end of the year. Mr. Priestap, a 20-year veteran of the bureau, worked on organized crime and drug cases in Chicago before rising through the national security ranks of the agency after the terrorist attacks...