[2] She was appointed interim sheriff by Mayor Ed Lee pending an ethics investigation of the elected sheriff, Ross Mirkarimi, who was charged with domestic violence battery and later agreed to a plea bargain in which he pleaded guilty to misdemeanor false imprisonment.
Hennessy is a native of San Francisco and a graduate of Lowell High School.
In 2006, she became the deputy director of the San Francisco Department of Emergency Services and Homeland Security, and from 2008 to 2011 she was San Francisco's director of emergency management.
[3][5] On November 17, 2014, Hennessy announced that she would run for sheriff of San Francisco.
[7] Hennessy defeated incumbent Sheriff Ross Mirkarimi with 61 percent of the vote.