Garcetti had previously endorsed Lacey, but hinted this summer that he was re-evaluating his stance on the race.
"I am proud to endorse George Gascón for District Attorney. He knows how to promote public safety through partnerships with and beyond law enforcement," Garcetti said in a statement Sunday.
"George Gascón will help our county shift the burden from the criminal justice system and jails toward diversion, intervention, and re-entry programs that save money and save lives," the statement said.
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Gascón says he is honored to have Garcetti's support, adding that he's ready to inspire big change.
In the wake of nationwide protests calling for criminal justice reforms after the deaths of George Floyd and Breonna Taylor, many officeholders have walked back their endorsements of Lacey.
Gascón is a former San Francisco D.A. and Los Angeles police officer.
Gascón has said he would abolish cash bail and opposes the death penalty. He co-authored Proposition 47, which reclassified some nonviolent felonies to misdemeanors and allowed defendants to renegotiate punishments for past convictions.
He's been criticized for Prop 47, which some in law enforcement said has led to an increase in crime.
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Lacey, the two-term incumbent, supports bail reform and the death penalty, but she has been criticized during her tenure for the racial disparity in county jails, high incarceration rates and for failing to prosecute law enforcement officers for fatal shootings and other on-duty uses of force.
Lacey dismissed the mayor's endorsement. She issued the following statement:
"This is just George Gascón rolling out another politician endorsement to distract from his disastrous performance in last night's debate. Gascón last night was asked for the first time about his policy advisors who called for abolishing prisons and who called for burning down the city, and his response was to defend their statements and confirm that they are still on his 'allstar' policy team. LA County voters care whether they're safe, not about politician endorsements."
The L.A. County District Attorney's office is the largest local prosecutor's office in the United States.
City News Service contributed to this report.