McDonnell, who is the Los Angeles Police Department's former "second in command," may have an advantage in the campaign. Unlike most of the leading candidates, McDonnell has never worked at sheriff's headquarters and he's never run for office before.
But he's already boasting endorsements from some of the most popular and wealthy people in the city as he begins his first-ever run.
McDonnell wants to bring an outsider's perspective to the largest sheriff's department in the country.
"I think it's tremendous advantage. Because I think when you're brought up in a strong culture, you tend not to see alternatives. You tend to see things being done the way you've always done them," said McDonnell.
McDonnell previously said he wasn't going to run against Sheriff Lee Baca, but Baca's surprise resignation announcement last week changed everything.
"It certainly changes the whole dynamic as far as the amount of money you have to raise, the amount of time you would have to expend to be a competitor and then what ultimately your chances would be for success in the end," said McDonnell.
McDonnell joined Long Beach police in 2010 after serving the LAPD for 29 years.
LAPD Chief Charlie Beck is among his supporters for sheriff, in addition to current District Attorney Jackie Lacey, former District Attorney Steve Cooley and businessman Rick Caruso.
"For them to be in a position to be able to validate my candidacy means the world to me," said McDonnell.
McDonnell investigated the sheriff's department as a member of the Citizen's Commission On Jail Violence, alongside Miriam Krinsky.
"Chief McDonnell is really the only one thus far in the race that can bring that outsider perspective while also bringing the leadership skills of having run a police department," said Krinsky.
LAPD Detective Lou Vince is the only other "outsider" in a field that includes former Undersheriff Paul Tanaka, former Commander Bob Olmsted, retired Lt. Patrick Gomez and current Assistant Sheriff Todd Rogers.
"I'm a novice to this. I've never run for office. I'm not a politician. My goal is to try and showcase what I bring to the table. I'm not going to try to run down any of my opponents. Everybody works hard. They're all cops at the end of the day," said McDonnell.
Vince and Olmsted on Monday criticized McDonnell for getting into the race after Baca got out of it.
McDonnell shrugged off those comments. He says he will stay in his current job as he campaigns for the primary election next June.
