Williams will succeed his former boss, Lynne Abraham, who decided not to run for office again. She'd held the post since 1991.
Williams spent 10 years working as an assistant prosecutor under Abraham.
He unsuccessfully challenged her in 2005 and later became Philly's inspector general.
Last year, Williams defeated four other Democrats to earn his party's nomination in the May primary and then he easily won the general election.
Williams steps into the job at a tough time -- Philly holds the highest violent crime rate among big cities, according to the Philadelphia Inquirer, and the nation's lowest felony conviction rate, winning only one in five cases.
"Unquestionably, we have to do a much better job," Williams told the Inky. "My administration won't be Lynne Abraham-lite," Williams said about the low conviction rate.
Williams says he plans to set up a way to measure performance within the city's judicial system, pressure judges to make parts of the court process more efficient for police and prosecutors and push for the death penalty in fewer cases.
Williams, 42, will be inaugurated in a ceremony at the Kimmel Center in center city.