San Francisco District Attorney George Gascón announced Tuesday that he will not run for re-election, bringing to a close an eight-year run as one of the country’s most progressive top prosecutors.

Citing family obligations, including plans to care for his ailing 90-year-old mother in Southern California, Gascón said he will not seek a third term. His term expires at the end of 2019.

“At this time, I simply cannot be the son I want to be and seek a third term,” Gascón said in a statement. “My career means a great deal to me. But success in the world with a family in chaos is not a choice I am willing to make.”

Gascón faced what was sure to be a contentious and lengthy fight for re-election next year. Suzy Loftus, assistant chief legal counsel in the Sheriff’s Department and a former president of the Police Commission, announced her bid for the job last month and has already secured the endorsement of Mayor London Breed.

Joseph Alioto Veronese, a onetime member of the San Francisco Police Commission, and Leif Dautch, a state deputy attorney general and a favorite of the San Francisco police union, have both thrown their hats in the ring.

Gascón was appointed district attorney in 2011 after Kamala Harris left the position to serve as state attorney general. He had been San Francisco’s police chief from 2009 to 2011 after being tapped from his position as police chief in Mesa, Ariz., by then-Mayor Gavin Newsom. Prior to that he was an assistant police chief for the Los Angeles Police Department.

Over his two terms as district attorney, Gascón has fought for change in the criminal justice system at the local and state levels. His efforts created foes on both sides of the political aisle.

He famously clashed with the Police Department’s rank and file, command staff and former Chief Greg Suhr over initiatives on police use of force and legislation reducing punishment for certain drug and property crimes, namely Proposition 47.

Gascón this year faced wide criticism from law enforcement groups as the only law enforcement official in the state to support failed AB931, which would have created a stricter standard for when police officers can use deadly force.

His decisions to not prosecute police officers in high-profile killings — like the 2015 fatal police shooting of Mario Woods in the city’s Bayview district — have drawn sharp rebukes from police-brutality protesters and some communities of color in San Francisco.

Every Friday afternoon, a handful of demonstrators gathers on the steps of the Hall of Justice, calling on Gascón to step down in wake of his decisions to not prosecute officers who use deadly force.

“The job of District Attorney is an enormous responsibility. It requires doing the right thing, even when it is unpopular, perhaps especially when it is unpopular,” Gascón said in his statement.

Among his efforts to bring change, Gascón expanded his office to fight crime by focusing on analytics and partnerships. His office participated in numerous task forces and started the Crime Strategies Unit, a team of prosecutors and analysts who work with the Police Department to identify prolific criminals and crime hot spots.

He cited a dramatic drop in violent crime, specifically homicides, under his tenure as police chief and district attorney. Killings in San Francisco dropped by 50 percent during Gascón’s first year as chief.

His biggest case as district attorney, though, ended in a high-profile acquittal as the eyes of the country watched.

A jury in November found Jose Ines Garcia Zarate not guilty on homicide charges in the 2015 killing of Kate Steinle on San Francisco’s Pier 14.

A team of attorneys from the public defender’s office argued that Garcia Zarate, an undocumented immigrant from Mexico, accidentally pulled the trigger on a stolen gun he found on the pier.

President Trump, who cited the case during his election campaign to argue for stricter immigration policy, blasted San Francisco and its sanctuary city policy following the verdict.

While the case drew brief but intense national criticism, Gascón has faced more sustained pressure locally as San Francisco’s property-crime rates — especially auto burglaries — spiked to unprecedented levels in recent years. Suhr and other police officials blamed Prop. 47 for the problem.

Chief Bill Scott has avoided finger-pointing and public clashes with Gascón’s office since becoming chief last year.

“The district attorney made a decision that was the best decision for his family,” Scott said Tuesday. “We still have a lot of work ahead together for the remainder of his term, and we look forward to continuing to partner with him to make our city safer.”

Gascón has also been active in efforts to reform the state’s cash bail system, though in recent months he has fought against more radical changes in the system. He was an early advocate of the Public Safety Assessment tool now used by the courts to determine a defendant’s risk upon release.

But when the state’s cash bail system was nearly scrapped altogether following a court of appeals ruling in the case of robbery defendant Kenneth Humphrey, Gascón found himself on the other side of the fight, advocating for a need for money bail in certain cases.

More recently, Gascón announced he would wipe thousands of marijuana convictions off the books in San Francisco following state Proposition 64, which legalized pot in California. Other major U.S. cities and Bay Area jurisdictions followed with similar initiatives.

Public Defender Jeff Adachi, his natural adversary in the courtroom, said he will be sad to see Gascón go.

“He’s been a tremendous person to work with,” Adachi said. “He’s very strongly committed to improving the system.”

After completing his term, Gascón said he plans to spend more time with his family in Southern California, where his family immigrated to from his native Havana when he was a boy.

“To do well requires a serious commitment of time and energy,” he said. “I have been able to do that because of the support of my family. The women in my life have made countless sacrifices for my career.”

Evan Sernoffsky is a San Francisco Chronicle staff writer. Email: esernoffsky@sfchronicle.com Twitter: @EvanSernoffsky