New York City mayor-elect Bill de Blasio announced on Thursday that the city’s former top cop is returning to the post.

De Blasio appointed William Bratton, a 66-year-old Boston native, to succeed Ray Kelly, saying the "proven crime-fighter" with decades of law enforcement experience will keep the nation's biggest city safe.

Bratton first served as NYPD commissioner under Mayor Rudolph Giuliani from 1994 to 1996. He has also led the Boston and Los Angeles police departments and has worked at several private security firms since 2009, when his 7-year LAPD tenure ended after expanding the department's use of stop-and-frisk tactics.

Bratton, known for his outsized personality and fondness for the limelight, has long emphasized the broken-windows theory of police work: that criminals who commit small crimes, such as vandalism, also commit more serious crimes.

Bratton helped spearhead the use of CompStat, a data-driven system of tracking crimes that allows police to better allocate their resources to high-crime areas. The real-time system is still used today.

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Crime immediately plummeted under Bratton, who benefited from an influx of new police officers. The year before Bratton took office, there were 1,946 murders citywide; by contrast, in 1996 there were 983, the first time it had dipped under 1,000 since 1968.

But Bratton frequently fought with Giuliani over who deserved the lion's share of the credit. He resigned after two years and later became the top cop in Los Angeles, where violent crime declined by 26 percent during his first three years.

"Bill Bratton is a proven crime-fighter," de Blasio said in a statement announcing the appointment. "He knows what it takes to keep a city safe, and make communities full partners in the mission. Together, we are going to preserve and deepen the historic gains we've made in public safety — gains Bill Bratton helped make possible."

Officials at the New York Civil Liberties Union said the organization will work to ensure that "fundamental changes" are made to the NYPD and its 34,500 uniformed officers during Bratton's return.

"The mayor won election on the promise to close the book on the tale of two cities, and New Yorkers from all backgrounds have made clear that we will hold him to that promise," NYCLU Executive Director Donna Lieberman said in a statement. "It’s a challenging task, but we are optimistic we will begin to see an era where members of all communities can trust the police, and where the police respect the rights and dignity of all New Yorkers."

De Blasio, who has vowed to improve police-community relations, takes office on Jan. 1, following Mayor Michael Bloomberg. Bratton will replace Kelly, the NYPD's longest-serving commissioner who has also been praised as one of the city's most effective. Besides overseeing the historic reduction in crime, Kelly dramatically bolstered the NYPD's counterterrorism and surveillance capabilities after the Sept. 11 attacks. The NYPD has foiled more than a dozen terror plots since but has come under criticism for its surveillance of Muslim communities, revealed in a series of Associated Press stories.

In New York City last year, there were a record-low 414 murders and this year is on pace to be even lower.

De Blasio also appointed Anthony Shorris as First Deputy Mayor, the city's second highest-ranking City Hall official. Shorris, a former executive director at the Port Authority of New York and New Jersey, has also served as deputy education chancellor and former finance commissioner for then-Mayor Edward Koch.

FoxNews.com's Joshua Rhett Miller and The Associated Press contributed to this report.