During a press conference Friday, Mayor Karen Bass made the formal announcement and called McDonnell a "nationally recognized thought leader on public safety."
"Chief McDonnell is a leader, an innovator and a change-maker and that's what we need in L.A.," she added. "I'm appointing Chief McDonnell because he and I have aligned in our desire to change the direction of Los Angeles by preventing crime in the first place, responding urgently when crime takes place, and to hold people accountable and bring down crime."
The appointment follows the retirement of Michel Moore earlier this year. Moore's tenure was marked by greater scrutiny into excessive force and police killings of civilians in the nation's second-largest city. Dominic Choi had been serving as the interim chief since then.
As the incoming chief, McDonnell will have to make sure the department is ready for the additional security challenges of the 2026 World Cup and the 2028 Olympics.
McDonnell was elected L.A. County Sheriff in 2014 to oversee the largest sheriff's department in the U.S. Before that, he spent 29 years in the LAPD and served as Long Beach's police chief for almost five years.
"I began the LAPD Academy 43 years ago... and I pause when I say that because I just can't wrap my head around that. That is a long time," McDonnell said Friday. "I love this city and I understand the modern-day challenges our officers face in trying to protect it."
At the LAPD, McDonnell held every rank from police officer to second-in-command under former LAPD chief Bill Bratton. During that time, he helped implement a federal consent decree imposed on the department largely as a result of the Rampart scandal, a corruption case involving rampant misconduct within the anti-gang unit.
When he was elected county sheriff, he inherited a department in the wake of a jail abuse corruption scandal that led to convictions against his predecessor, longtime Lee Baca, and more than 20 other officials.
The LAPD has faced criticism through the years over its response to the George Floyd protests and several high-profile shootings by officers. It has struggled to get rid of bad cops while also struggling to recruit as more officers leave its ranks than are coming in.
McDonnell touched on some of his priorities as the new leader of the department.
"In simplest terms, my goals are to enhance public safety, to grow our department back to full strength... to strengthen public trust, the foundation of all we do, to further develop community relationships and to be able to take that to new levels. To ensure respectful and constitutional policing practices in all that we do," the new LAPD chief said.
While there was debate over whether Mayor Karen Bass would choose an "insider" or "outsider" who would shake things up and challenge the way things were done within the department's insular culture, the mayor ended up with three candidates all with decades of experience at the LAPD.
The other two candidates sent to Bass, who made the final selection, were Deputy Chief Emada Tingrides and former Assistant Chief Robert "Bobby" Arcos, both reported by the Los Angeles Times as finalists for the position.
The Associated Press contributed to this report.