Garcetti and his wife Amy were cheered as they arrived for the public swearing-in ceremony. Late night host Jimmy Kimmel acted as the master of ceremonies and warmed up the crowd up by joking about the heat.
"I 'd just like to point out that the heat today is the responsibility of the previous administration," said Kimmel.
Garcetti spoke for about 10 minutes and promised he'll be a "back to basics mayor."
The oath of office was administered by Kenia Castillo, an eighth grader from Burbank.
Castillo first met Garcetti when she was 4 years old at an event rallying support for better working conditions for janitors, like her mother. Castillo later reunited with Gacetti during his mayoral campaign, and told him she would become a campaign volunteer because he had been there for her family in the past.
Garcetti told the crowd he plans to get to work right away on two big issues facing L.A., jobs and growth.
"These times demand a 'back to basics mayor' focus above all on our economy and jobs, and that's what I intend to be," said Garcetti.
The 42-year-old Ivy Leaguer plans to nurture small businesses and attract new business to the city.
"We have to start with the most basic task of all through, hanging a bright neon sign on our city that says 'Los Angeles is open for business,'" said Garcetti.
Also sworn in was Ron Galperin as controller, Mike Feuer as city attorney and eight members of the City Council, including six newcomers.
The six new City Council members include Mitch O'Farrell, succeeding Garcetti in the 13th District, which stretches from Hollywood to Glassell Park, where O'Farrell lives; Curren Price, who is leaving his seat in the state Senate to replace Jan Perry in the Ninth District; Bob Blumenfield, succeeding Dennis Zine in the Third District in the West San Fernando Valley; Gil Cedillo, succeeding Ed Reyes as the new First District council member, representing Northeast Los Angeles; Felipe Fuentes, succeeding Richard Alarcon in the Seventh District in the Eastern San Fernando Valley; and Mike Bonin, succeeding Bill Rosendahl in representing the 11th District, which includes Brentwood, Pacific Palisades, Venice and Westchester.
The evening ended with a free public party in downtown's Grand Park. The event included carnival games and music from Moby.
Garcetti officially becomes the mayor of Los Angeles when the clock strikes midnight and Democrat Antonio Villaraigosa's term ends.
City News Service contributed to this report.