CHICAGO — With the members of the City Council, former Mayor Richard M. Daley and Mayor Rahm Emanuel looking on, Lori Lightfoot was sworn in as Chicago’s 56th mayor on Monday, lauding the city’s resilience and diversity while also vowing to kill old-style Chicago politics.

“For years, they said ‘Chicago ain’t ready for reform,’” said Ms. Lightfoot, 56, a former federal prosecutor, addressing the crowd of thousands who had gathered in an arena on a blustery May day. “Well, get ready, because reform is here.”

Ms. Lightfoot’s inauguration made history. She is the city’s first openly gay mayor. When she won all of Chicago’s 50 wards in April, the city became the largest ever to elect an African-American woman as its mayor.

One of her first orders of business, Ms. Lightfoot said, would be ending an unwritten rule that gives aldermen unfettered zoning and permitting power in their own wards.