The transfer of power caps a transition period that began with McGreevey's stunning disclosure in August that he would resign because of a gay sex scandal.
Codey, 57, a Democrat, opted for a private swearing-in ceremony in deference to the circumstances of McGreevey's departure, aides said. The event at his West Orange home was closed to the public and the news media, except for an Associated Press photographer.
"I never wanted to be governor under these circumstances. And this is not the time for a big ceremony or a big swearing-in," Codey said outside his home minutes after the brief ceremony.
But he added, "I'm looking forward to governing and bringing back calm, peace and a sense of harmony to the state of New Jersey."
He was sworn in by state Senate Minority Leader Leonard Lance, a Republican from Hunterdon County. Besides a few staff members and state police security, the only other people at the ceremony were Codey's wife, Mary Jo, their two sons and Monsignor Michael Kelly of Seton Hall Prep.
McGreevey's announcement that he had an extramarital affair with a man and would resign threw the state's executive branch into turmoil and put Codey in line to assume the governor's job with 14 months left in McGreevey's term.
Because New Jersey is one of eight states without the position of lieutenant governor, Codey will wield the clout of both governor and Senate leader for a time, filling the governor's term that ends in January 2006.
Although he was not constitutionally obligated to, Codey recited the oath that specifically empowers him as Senate president to perform the duties of governor. A signed version of his oath will be filed with the secretary of state, making Codey governor at 12:01 a.m. Tuesday.
Aides said McGreevey's last day in office is expected to be quiet, spent with family away from Trenton. He is not expected to grant any pardons or conduct state business.
On Friday, a pair of moving trucks carted the governor's belongings away from the Statehouse. McGreevey is reportedly moving to an apartment in Rahway while his wife moves to a home in Springfield, where she plans to live with the couple's 2-year-old daughter.
Codey, first elected to the Legislature in 1973, plans to remain in his home, declining to move to the governor's mansion in Princeton.
He said last week that he has not ruled out a run next year for a full term as governor, although U.S. Sen. Jon Corzine, a popular politician with vast financial resources, is expected to pursue the Democratic nomination. Some of the state's highest-profile Republicans also have been lining up to run for the job.
Codey plans to make ethics reform a top priority. Ending the practice of awarding government contracts in exchange for campaign contributions has topped lawmakers' agendas over the past few months.
McGreevey faced serious questions about the ethics of his administration, including the hiring of his alleged lover as his homeland security adviser.
Codey, a longtime champion of mental health concerns, added that one of the first things he will do is form a task force to look at ways of improving New Jersey's care for mentally ill people.
Codey also has been a longtime proponent of putting slot machines at the Meadowlands horse-racing track, a move that could help close a projected $4 billion budget deficit but could also reshape gambling in the state, where Atlantic City casinos have long been the top players.
In his farewell address as governor last week, McGreevey highlighted his own achievements in office, including reforms of the state's child welfare agency, environmental protections and benefits for domestic partners.
The governor also said he does not apologize for being "a gay American" but does regret having allowed his personal feelings to impact his decision-making.
"I am sorry that I have disappointed the citizens of the state of New Jersey who gave me this enormous trust," said McGreevey, who hasn't announced his future plans, which are said to involve some type of public service. "I don't look back with bitterness, anger or sorrow. I look forward to seeking knowledge, a journey of self-discovery."