In 2002, he followed Christie to the office of the United States Attorney for the District of New Jersey, where he led a number of the office's high-profile public corruption cases, including the one against former State Senate President John A. Lynch, Jr. He served as Counsel to the U.S. Attorney, Chief of the Public Protection Unit, and Executive Assistant U.S. Attorney. He left in 2009 to become a partner in the firm of Wolff & Samson.
In 2009, after Christie was elected Governor of New Jersey, Chiesa headed his transition team. Christie then named Chiesa his chief counsel. In June 2010, Christie sent him to speak to Republican Assemblyman Michael Patrick Carroll, to persuade him to drop his opposition to the budget because it cut proportionally more aid to suburban schools than to urban ones.
In February 2012, he helped deliver $837.7 million to distressed homeowners of New Jersey from a settlement with major banks. The state had 10.6% of homeowners who are 90 or more days delinquent on their mortgage, the third-highest percentage in the nation at the time.
In April 2012, he announced the arrest of three men accused of theft at several Home Depot stores across five states: New Jersey, Pennsylvania, Delaware, Maryland, and New York. They were "under-ringing" their purchases at self-checkout machines and were charged with more than 500 illegal transactions totaling more than $100,000. In the same month, he announced the arrests of 27 people in a major child pornography incident, that required the involvement of more than 100 law enforcement officers for "Operation Watchdog." He also filed a lawsuit against John Kot and Gabriel R. DaSilva of leading home improvement companies for defrauding people and breaking several laws.
Chiesa resigned as Attorney General on June 6, 2013 and Executive Assistant Attorney General John Jay Hoffman became acting Attorney General. Chiesa was sworn into the Senate on June 10, 2013 by Vice President Joe Biden. Chiesa was the first Republican senator to represent New Jersey since 1982 when then-Governor Thomas Kean appointed Nicholas F. Brady to the Senate in order to fill a vacancy.
He used his time in the Senate to try to draw attention to the issue of human trafficking and, according to The Washington Post, voted with his party 84% of the time. He left the Senate on October 31, 2013 when Newark Mayor Cory Booker, who won the special election, was sworn in. Chiesa recorded a tenure of 129 days, the fourth-shortest of the 65 U.S. Senators who have served in New Jersey's history.
Chiesa ruled out the possibility of running for the seat in the regularly-scheduled 2014 election, but said he would consider running for office in the future.