Lee, the city administrator, was sworn in Tuesday after the Board of Supervisors unanimously named him to serve out the remaining year of Gavin Newsom's term, the San Francisco Chronicle reported. Newsom is now lieutenant governor of California, and Lee says he will not run for mayor in November.
"I hope to move us past the labels that have pigeonholed us at City Hall," Lee told the crowd in the rotunda.
He fills the first mayoral vacancy since then-Supervisor Dianne Feinstein was appointed after George Moscone was assassinated in 1978.
Feinstein's daughter, San Francisco Superior Court Presiding Judge Katherine Feinstein, administered the oath.
Board President David Chiu said Lee's rise was "not just about the Chinese American community or the Asian American community. This is about the American Dream, the idea that anyone, from any background ... can come here and someday be at the very top of what our community is about."
"I'm not going to change," Lee said. "I'm going to open up that Room 200 daily. I present myself as a mayor for everyone."