"My consultants tell me that while I may close the gap between the two of us, I will not be able to make up enough based on the trending of the ballots to win this election," Lacey said at an emotional press conference attended by her staff and supporters.
According to the most recent preliminary results released by the county registrar-recorder's office, Gascon had 53.7% of the vote to Lacey's 46.2% as of late Thursday afternoon, giving the former San Francisco DA a lead of 229,022 votes.
In a statement, the registrar's office said about 791,200 votes from Tuesday's election were yet to be counted. These included vote-by-mail ballots, conditional voter registration ballots and provisional ballots cast at voting centers.
The race created an unusual dynamic in which Gascon, a former patrol officer and police chief, was fiercely opposed by law enforcement unions and Lacey, the first woman and Black person to run the office, was criticized by Black Lives Matter activists.
Gascon vowed to bring sweeping changes to the office such as jailing only dangerous criminals and holding police officers accountable for unjustified killings.
Gascon's message resonated with voters after a summer of activism over the killing of George Floyd brought attention to the issue of police brutality and racial inequality.
"We will stop the death penalty immediately,'' Gascon said after declaring victory and thanking his supporters at a virtual press conference. "We will begin to unwind current cases that are on the death penalty track. We will immediately stop prosecuting children as adults."
Adding to the number of reforms he promised to implement after he takes office, Gascon said he had "committed personally to reopening some cases involving law-enforcement uses of force, and I always made it clear that there may be other cases as well, and I am committed to doing so. We're going to be taking a very different look at the way that we handle people with mental health problems.''
When a reporter asked how many members of his team will be joining him in the L.A. County DA's office, the former LAPD assistant chief of police said he was evaluating "both internal and external" personnel.
Gascon also said he intends to change the way police officers and sheriff's deputies are held accountable.
"We have to get away from police departments investigating themselves," he said, pointing to San Francisco's Independent Investigations Bureau may be a possible model for reform.
Gascon insisted that his significant electoral lead over Lacey in the election gave him a mandate to make changes as head of the largest local prosecutorial office in the country.
"This was not a close race," Gascon said. Although former Vice President Joe Biden and Sen. Kamala Harris are still in a tight race with President Donald Trump and Vice President Mike Pence, Gascon said he is "very, very committed to working with the Biden-Harris administration ... when it comes to criminal-justice reform."
As incumbent who served two terms, Lacey's tenure was marked by protests from progressives who felt she was not aggressive enough in prosecuting police officers and sheriff's deputies involved in civilian deaths.
Outside of Lacey's office at the Hall of Justice, BLM Los Angeles has held weekly protests for the past three years, demanding accountability from authorities.
The activists' chants often include "Bye, Jackie!" "Jackie Lacey must go!" and "Prosecute killer cops!"
One of the organizers of the protests outside Lacey's office compared those demonstrations to the 381-day Montgomery bus boycott in Alabama in the mid-1950s.
"So we are in that tradition, we are in that pattern," he said. "We have been here for over three years now. I've been looking at pictures that I took back in 2017. And it is our consistency and regularity that has helped sustain us.
"But this is a victory for the families," he said, referring to those whose loved ones have died in incidents involving law enforcement officers.
The Associated Press contributed to this report.