LOS ANGELES — Eli Broad, a billionaire who has been a dominant figure in this city’s cultural, philanthropic, business and political world for almost 50 years, announced on Thursday that he is stepping down from public life, saying that it was “time to move on.”

“I am retiring,” Mr. Broad, 84, said as he ushered two reporters into his 30th-floor office at the Broad Foundations here in Century City. “Now. Right now. I am just tired. I want to spend more time with my family. Catch up on my reading.”

“We have been thinking about this for a long time,” he added. “The time has come.”

It is difficult to overstate Mr. Broad’s importance to Los Angeles. His vast fortune has shaped the city, from its arts and medical worlds to its reinvigorated downtown. He has lived here for 52 years, since coming from Detroit as a young entrepreneur. He has been a confidant of mayors and governors, an aggressive advocate for charter schools and a heavy contributor to medical causes, particularly stem-cell research. He has given away or pledged $4 billion in his life. The endowment for the Broad Foundations, his main philanthropic arm, is $2.5 billion. His current net worth is $7.3 billion, which he made in construction and insurance.