[4] His tenure was notable for the development of prisoner education and rehabilitation programs, by construction of three major jail facilities and by expansion of powers of the Office of Sheriff.

Hennessey graduated with a degree in history from St. John's University (Collegeville, Minnesota) in 1970 and then moved to San Francisco to attend law school.

[5] Following graduation from law school, Hennessey took a temporary job in the San Francisco Sheriff's Department as Legal Counsel to Sheriff Richard Hongisto (December 1973 – June 1974) and then joined the newly created University Year for Action (UYA) program providing a variety of social services to prisoners in the San Francisco county jail.

[8] Six weeks after taking office, four federal bank robbers escaped from the city's oldest jail, although all were recaptured within 24 hours.

A gun was smuggled into the jail's high security unit and thirteen dangerous prisoners escaped from the downtown Hall of Justice.

[10] A year later, one of the Department's own deputies helped a leader of the Hells Angels escape by hiding him in a laundry cart and pushing him out to freedom via the jail's freight elevator.

Hennessey and Marcum became lifelong friends and their friendship resulted in many controversies, including a vote of no confidence by the Deputy Sheriff's Association and a demonstration by deputies when, in 1993, Hennessey promoted Marcum to the position of Assistant Sheriff, the third highest position in the Department.

His popularity is largely attributed to his progressive leadership in introducing innovative rehabilitation programs in the jails, aggressive hiring within San Francisco's minority communities, and improving the professionalism of the Department.

"In my 32 years as sheriff, I presented dozens of cases involving deputies accused of everything from sexual assault to smuggling contraband to falsifying time sheets to three different district attorneys — and not one case was ever charged," he told San Francisco columnists Matier and Ross.

See Title 24, Section 1231.2.10 [21] Another controversial effort by Hennessey was to introduce AIDS education to both inmates and staff in the jails.

Hennessey began this program very early in the AIDS crisis which hit San Francisco's large gay population especially hard.

He is widely quoted in a recent history of the local punk rock scene, Gimme Something Better, by Jack Boulware and Spike Tudor.