The 67-year-old Hackett was among 12 people, including 11 public officials, arrested last fall on federal bribery charges.
The Democrat resigned from the Assembly days after his arrest. His mayoral term was set to expire on July 1.
"He apologizes to the citizens of Orange," Hackett lawyer John A. Azzarello said after Hackett pleaded guilty in the morning to a single federal charge of attempted extortion under color of official light.
Hackett admitted he took a $5,000 bribe to steer a city contract to an insurance broker. The broker, however, was an undercover informant working for the FBI.
"They wanted me to influence one of the officials in the city," Hackett told U.S. District Judge Jose L. Linares.
Azzarello said that Hackett hesitated before taking the cash, which was stuffed in an insurance brochure. "That's the moment in time that the mayor made a grave error in judgment," the lawyer said.
Several hours later in state Superior Court in Newark, Hackett pleaded guilty to an official misconduct charge brought by state prosecutors, who claim he collected more than $5,700 in fictitious meal expenses.
U.S. Attorney Christopher J. Christie said, "It is an awful turn of events when people who hold multiple elective positions use all of the authority given to them to line their own pockets." He added, "Mr. Hackett's criminal conduct betrayed not only the people of the City of Orange but the people of the state of New Jersey."
State Attorney General Anne Milgram said, "Mayor Hackett took an oath to serve the city of Orange with honesty and integrity, but instead he systematically stole thousands of dollars from the city at a time when it was struggling to meet its financial obligations and the needs of its residents."
Under a plea deal reached with state and federal prosecutors, Hackett would face about 18 to 24 months in federal prison. Hackett also agreed to repay the $5,000 bribe and must also make restitution to Orange.
Hackett remains free on bond pending his sentencing in federal court in Newark on Sept. 15.
