Cory Booker, who became a rising star in the Democratic Party as mayor of Newark, has been sworn in to the U.S. Senate, becoming New Jersey's first black senator.

Booker, 44, won a special election Oct. 16 to fill out the term of the late Sen. Frank Lautenberg. Vice President Joe Biden conducted the ceremony in the Old Senate Chamber.

Booker is now one of two black senators currently serving; Republican Tim Scott of South Carolina was elected last year. There have been just nine in U.S. history, and he is only the fourth to be popularly elected.

Booker's career has finally ascended to the national stage, but his profile has been there for years. He was dubbed a "rock star mayor" by his friend Oprah Winfrey, has 1.4 million Twitter followers and is a fixture on late-night talk shows.

Booker tapped Wall Street to donate philanthropic funds to Newark and persuaded Facebook founder Mark Zuckerberg to give $100 million to its school system in 2010.

He runs in celebrity circles, raising money in Silicon Valley, New York City and Los Angeles, where actor Matt Damon co-hosted a campaign fundraiser for him in September.

Despite the high-profile connections and his status as the most recognizable Democrat in New Jersey, Booker will have to adjust to being the most junior of 100 senators. Yet he is starting off his tenure with high-profile meetings Thursday with Senate Majority Leader Harry Reid and President Barack Obama.

Booker formally resigned as mayor of Newark on Wednesday afternoon. It was effective at midnight.

"While I am leaving one position, I am not leaving Newark," Booker said in his resignation letter. "I am proud to be able to now represent Newark and our entire state as a United States senator. My level of dedication, passion and service will not falter as I serve New Jersey."

Booker leaves Newark with a mixed legacy. The downtown has markedly improved, with high-rise corporate headquarters, new housing and restaurants popping up and a Whole Foods supermarket scheduled to open in 2016. Parks have been cleaned up throughout the city and violent crime is down in most cases.

Many residents, however, feel that areas outside downtown have been forgotten. Vacant lots and blighted homes dot neighborhoods. The city's unemployment rate is 14.2 percent, half of its 277,000 residents live below the poverty level and the high school dropout rate is just under 40 percent.

After Lautenberg's death, Gov. Chris Christie appointed former Attorney General Jeffrey Chiesa to fill the position until the special election. Minority Leader Mitch McConnell bade Chiesa farewell on the Senate floor Wednesday afternoon.