Williams, 42, maintained a commanding lead of nearly 3-1 over Republican Mi chael Untermeyer, 58. The election marks the first change of leadership in nearly two decades for the office run by District Attorney Lynne M. Abraham.

Williams, a former assistant prosecutor and city inspector general, ran unsuccessfully for district attorney against Abraham in 2005. His second attempt was the charm.

Nationally, fewer than 40 of approximately 2,000 elected prosecutors in America are black or Latino, said Suffolk University Law School professor Jeffrey Pokorak, who since 1998 has studied the race of elected district attorneys for law review articles about capital punishment.

For Williams, the road to a win was not without bumps. It started last winter with a bruising five-man Democratic primary. After an opponent brought a lawsuit challenging the propriety of Williams' campaign spending, a judge of Election Court removed him from the ballot. Thirteen days later, he was reinstated by Commonwealth Court.

The legal attack on his candidacy "backfired," Williams said at the time. "I got all kinds of free publicity from that."

On the campaign trail, he billed himself as the most experienced candidate, citing his 10 years as an assistant prosecutor, including assignments running the repeat-offender and Municipal Court units. After leaving the District Attorney's Office in 2003, he was appointed city inspector general for two years under Mayor John F. Street.

At candidates' forums across the city, he reminded prospective voters that "Philadelphia leads the nation in the rate of homicide by handgun" and that "the leading cause of death for black men under 35 in Philadelphia is homicide."

He said "the justice system is broken" and the District Attorney's Office needs reform.

Pitching his slogan, "A new day. A new D.A.," he spoke of plans to decentralize the operation by assigning prosecutors to neighborhood police districts, diverting minor offenders into community-based drug and alcohol treatment programs, and providing "better care and support" for victims and witnesses.

He emerged victorious with 42 percent of the vote in the May 19 primary.

As a Democrat, Williams benefited from the huge registration advantage of his party, which outnumbers Republicans in the city by more than 6-1. Endorsed by nearly two dozen unions, including the 14,600-member Fraternal Order of Police lodge, Williams also enjoyed a strong organizational advantage, which helped get his voters to the polls on a day when the eleventh-hour strike by SEPTA, coupled with the traditionally low rate of participation in an off-year election, pushed overall voter turnout into the low teens.

Teaming with the Democratic City Committee and the Black Clergy of Philadelphia and Vicinity, Williams went to court yesterday to try to win an extra hour of voting because of the impact of the SEPTA strike.

After a two-hour hearing, however, Judge Lori Dumas-Brooks ruled that the strike was not enough of a hardship to warrant extending the polling hours beyond 8 p.m.

As district attorney, Williams will lead an office of more than 300 assistant prosecutors and 250 support staffers.

His priority, he said, is to position the office to be more effective against gun crimes. One of the ways he plans to do that is by strengthening the Gun Violence Task Force, a partnership between the District Attorney's office and the state attorney general.

To head that effort, he is likely to hire Brian Grady, one of his opponents in the Democratic primary, who helped Williams craft policy papers over the summer.

Grady is a former assistant prosecutor whose "passion" and "ability to cut through the bs," Williams said recently, make him a good candidate to lead the task force.

Williams will take office Jan. 5. To coordinate the transition, he said, he has appointed Mark Aronchick, a lawyer prominent in Democratic Party circles, and JoAnne Epps, dean of Temple University's Beasley School of Law.

In another closely watched race, Controller Alan Butkovitz, a Democrat, easily beat his Republican challenger, Al Schmidt, by more than a 2-1 ratio.

Contact staff writer Michael Matza at 215-854-2541 or mmatza@phillynews.com.

Staff writer Jeff Shields contributed to this article.