Mayor London Breed started the ceremony saying the city needs accountability.
"Despite even being victims of crime and experiencing challenges with our criminal justice system, they have said time and time again. Yes, we want accountability. But we also don't want to abandon our values to reform our criminal justice system, so that it is fairer, and it provides the safety that we need in a city like San Francisco."
VIDEO: Mayor London Breed speaking ahead of Brooke Jenkins' swearing in as new SF district attorney
Mayor London Breed speaking ahead of Brooke Jenkins' swearing in as new SF district attorney
Mayor Breed said she didn't take the decision in selecting a new district attorney lightly.
"When meeting and talking to Brooke and hearing her understanding of the issues and the balance that she brings to the table. I am so proud that she stepped up. She put her career on the line, because not only does she care about safety and the future of this city for her own family she cared about it for all San Franciscans."
After being sworn in, Jenkins took to the podium. "Starting today, drug crime laws will be enforced in this city," she said.
"We can no longer turn a blind eye to the gun violence that is going on. In our black community here. The families of this violence deserve justice."
Jenkins is no stranger to politics or the San Francisco District Attorney's Office.
Jenkins is a former assistant district attorney, who quit her job and led the recall effort to oust Chesa Boudin.
She has been appointed by Mayor London Breed. Some say Breed said she chose Jenkins because of her experience as a prosecutor.
RELATED: SF mayor announces Brooke Jenkins new DA to replace Chesa Boudin
Brooke Jenkins named SF's new DA
Jenkins has said that she will "restore" accountability and consequences to our criminal justice system here in San Francisco.
On Friday, ahead of Jenkins' swearing in, former San Francisco District Attorney Chesa Boudin released a statement writing:
"It has been my honor to serve as San Francisco's District Attorney as we worked for a safer and more just city. I am proud of the work we did to serve victims and to bring much-needed reforms and accountability to our justice system. This is work I will continue in the months and years ahead. I have reached out to Brooke Jenkins to offer any assistance I can provide in ensuring a smooth transition. I am tremendously humbled by the strong, talented team my administration has built, which will continue to serve San Franciscans. I am proud of the new programs my administration has launched to promote safety and justice in our city despite unprecedented challenges. I am deeply grateful for the dedicated staff at the District Attorney's Office who work tirelessly every single day to promote justice for the people of San Francisco. Our work continues."
She quit her job in October after clashing with Boudin over his management style and what she considered lax policies toward criminals.
Boudin did away with cash bail.
VIDEO: Former SF DA predicted Brooke Jenkins would replace him
Former San Francisco District Attorney Chesa Boudin predicted Brooke Jenkins would replace him
He rarely used gang enhancements. And he vowed to not charge minors as adults.
RELATED: SF police, hotel council hope for change after voters recall DA Chesa Boudin
Jenkins is seen as someone who will could continue on as a progressive, but not go to the extremes her predecessor was criticized for.
Jenkins will hold the position until November when voters will decide the next DA in a special election.
She says she will run - and many think Boudin will as well.
VIDEO: SF's new DA Brooke Jenkins talks 1-on-1 with ABC7
SF's new DA Brooke Jenkins talks 1-on-1 with ABC7
ABC7 News insider Phil Matier says the move by Mayor Breed makes sense.
"She was looking for someone who could continue as sort of a progressive area but not go to the extremes that she saw Chesa Boudin do," said Matier.
On Friday, ahead of Jenkins' swearing in, former San Francisco District Attorney Chesa Boudin released a statement writing:
"It has been my honor to serve as San Francisco's District Attorney as we worked for a safer and more just city. I am proud of the work we did to serve victims and to bring much-needed reforms and accountability to our justice system. This is work I will continue in the months and years ahead. I have reached out to Brooke Jenkins to offer any assistance I can provide in ensuring a smooth transition. I am tremendously humbled by the strong, talented team my administration has built, which will continue to serve San Franciscans. I am proud of the new programs my administration has launched to promote safety and justice in our city despite unprecedented challenges. I am deeply grateful for the dedicated staff at the District Attorney's Office who work tirelessly every single day to promote justice for the people of San Francisco. Our work continues."