Andrews, who previously aspired to higher office, said the inquiry was not a factor is his resignation, which is effective Feb. 18. He is taking a job running the governmental affairs unit of a Philadelphia law firm where his wife has worked.
Andrews, 56, said during a news conference at his district office in Haddon Heights that he has not decided whether to register as a lobbyist when he starts his work at Dilworth Paxson. If he does, he will not be allowed under federal law to lobby members of Congress in the first year he is out of office.
Andrews said the law firm offered him a job last week.
A congressional ethics committee has been looking into Andrews' spending after reports that he was using campaign funds for frequent trips to California, where his daughter had a fledgling singing and acting career.
The investigation started after the Office of Congressional Ethics issued a report in 2012 highlighting several questionable trips Andrews took that were paid for or reimbursed by campaign accounts.
In June 2011, Andrews, his wife and two daughters flew business class to Edinburgh, Scotland, for the wedding of a "political consultant," according to the OCE report. After the wedding, the family flew on to London for several days.
The report highlights other questionable reimbursements, including some of the costs of a party that jointly celebrated Andrews' time in Congress and the high school graduation of one of the daughters. Some of these expenses were "not ordinary and necessary," according to the OCE report.
The report also says that Andrews justified paying for several trips to Los Angeles and meals for daughter Josie, now 19, by listing her as a volunteer campaign staffer. That claim was "not supported by the evidence obtained," the OCE report concludes.
An Associated Press analysis of his campaign spending in 2012 found Andrews had spent at least $97,000 in campaign funds on at least 18 trips to California from 2007 to 2011.
The House Ethics Committee was expected to announce it will no longer pursue the case once his resignation becomes official, because it lacks jurisdiction over ex-members.
Citizens for Responsibility and Ethics in Washington, a watchdog group, has long criticized Andrews' campaign spending practices. Melanie Sloan, its executive director, said she wants the committee to release its findings publicly when the case is closed.
At his news conference, Andrews said the investigation had "no role at all" in his decision to resign. He would not say whether the case should be made public, saying, "It's not my place to comment on the process."
Andrews ran statewide twice, losing a gubernatorial primary in 1997 and then mounting a brazen, unsuccessful primary challenge in 2008 against U.S. Sen. Frank Lautenberg.
The challenge against Lautenberg, who died last year at 89, ruffled the feathers of some in New Jersey's Democratic establishment.
Andrews, who grew up in the blue-collar town of Bellmawr and commutes daily by train to Washington, said he would not return to Congress if he lost that election. His wife ran in the primary for the seat that year and won. But after Andrews' primary loss, he returned to the ballot for the general election.
Andrews also was in the spotlight when he broke with his party by supporting the war in Iraq and worked with the George W. Bush White House on the wording of the resolution authorizing military action. He also had a key role passing President Barack Obama's health insurance overhaul in 2010.
Andrews said Tuesday that he would support state Sen. Donald Norcross, the brother of powerbroker George E. Norcross III, if he runs for the suburban Philadelphia seat.
The 1st District seat is one of at least two in New Jersey that will be open for this November's election. Rep. Jon Runyan, a Republican has announced that he will not seek a third term in the 3rd District.