George Santos, the disgraced former Republican congressman from New York whose penchant for lying led to one of the oddest sideshows in modern U.S. politics, pleaded guilty on Monday to wire fraud and aggravated identity theft and admitted to an array of other frauds and deceits.

While Mr. Santos’s decision will allow him to avoid a costly trial that was scheduled for next month, it ensures that he will face at least two years in prison.

The plea offers a coda to the tale of a political underdog who catapulted to Congress less than two years ago and then watched his career quickly unravel as his seemingly endless series of falsehoods came to light. Mr. Santos repeatedly insisted that he would defend his innocence in court, only to reverse course as the opportunity approached.

The Mr. Santos who appeared in court bore little resemblance to the shameless provocateur whose antics once grabbed headlines. His bravado was replaced by an ill-fitting jacket and words of contrition for several scams involving money laundering, making false statements to Congress and fraudulently claiming unemployment benefits.

“I accept full responsibility for my actions,” he told the judge as he admitted to participating in a scheme to “artificially inflate” the number of contributions to his campaign to win support from the National Republican Congressional Committee.

Speaking to reporters outside the courthouse, Mr. Santos was more voluble. His voice thick with emotion, he said that his plea was an effort to make amends for misrepresentations to supporters and the government, as well as “the lies I told myself over these past years.”

“I allowed my ambition to cloud my judgment,” he said, “leading me to make decisions that were unethical and — guilty.”

Mr. Santos’s sentencing is scheduled for Feb. 7, meaning he will not have to report to prison for at least six months. Guidelines call for him to serve between six and eight years in prison, though a judge could decide to be more lenient.

Mr. Santos also agreed to pay a total of $373,749.97 in restitution to a variety of entities, including donors whose credit cards he used without authorization and the state unemployment fund he fraudulently received benefits from.

He will be required to forfeit another $205,002.97 before his sentencing. Mr. Santos will need to come up with the money for his final plea deal to go through, and if he is unable to do so his property may be seized.

Prosecutors had been preparing to call as many as 40 witnesses, including Mr. Santos’s donors, co-workers, relatives and his co-conspirator and former campaign treasurer Nancy Marks. On Monday, prosecutors framed his admission of guilt as an initial step toward rebuilding the trust in government that Mr. Santos had eroded.

“Today, for what may seem like the first time since he started his campaign for Congress, Mr. Santos told the truth about his criminal schemes,” said Breon S. Peace, the U.S. attorney for the Eastern District of New York. “He admitted that he lied, he stole and he conned people.”

Mr. Peace added: “He is worthy only of infamy.”

Democrats immediately seized on the guilty plea to blame Republicans for backing Mr. Santos in the first place. But the outcome may also be welcome news to Republicans in New York, where the party is preparing to defend a half dozen swing seats that could decide the House majority.

The party already lost control of Mr. Santos’s seat, representing parts of Queens and Long Island, to Democrats in a special election in February, held after Mr. Santos was expelled. But a high-profile trial would have opened an unflattering window into the inner workings of Republican campaigns and cast a shadow over other races on Long Island and in the Hudson Valley.

Mr. Santos burst onto the national political scene in 2022 when he flipped New York’s Third Congressional District and helped his party win control of the House.

At the time, he was heralded as a new kind of Republican. Young, Latino and gay, Mr. Santos was a proud adherent of former President Donald J. Trump who embraced his attention-grabbing style of politics and widely debunked claims of election interference.

But even before he was sworn in, Mr. Santos’s lies began to unravel.

In December of 2022, The New York Times reported that much of his biography and aspects of his campaign’s finances appeared to have been fabricated.

Additional reporting from The Times and other news organizations revealed a bizarre array of lies: from inane claims of playing volleyball in college to fundamental deceptions about his education, résumé and cultural heritage.

Last year, Mr. Santos was charged with 23 felony counts for a number of schemes, including stealing money from donors and lying to the government to cover his tracks. He originally pleaded not guilty to the charges.

Despite the legal headwinds, Mr. Santos remained in Congress until late last year, when a report from the bipartisan House Ethics Committee revealed that he had not only defrauded donors, but had used the ill-gotten gains to buy designer goods and Botox.

Republicans joined Democrats to expel him from Congress in December, making Mr. Santos one of only six congressmen in the nation’s history to be kicked out of office. More than two-thirds of his colleagues in the House voted in favor of expulsion, including more than 100 Republicans.

For a while, Mr. Santos remained defiant and seemed to revel in the attention engendered by his fall from grace. He threatened retribution on Republican lawmakers who turned on him and began selling short, personalized video messages on the app Cameo, earning hundreds of thousands of dollars. He also began filming a documentary.

All the while, Mr. Santos dismissed his criminal case as a “witch hunt,” echoing Mr. Trump’s defense against his own legal troubles.

Prosecutors painted Mr. Santos’s campaign as so desperate for cash that it turned to lying, cheating and stealing to finance its operations. But they also asserted a pattern of self-dealing, which saw Mr. Santos repeatedly using his campaign account as a personal piggy bank.

On Monday, Mr. Santos admitted to all of the conduct he had once denied, from using one donor’s credit card information to steal $11,000 for his personal use to persuading other donors to contribute to what he said was a super PAC supporting his campaign.

Mr. Santos also admitted to falsifying campaign finance records to include a $500,000 loan to his campaign that never existed in order to qualify for financial and operational support from the national Republican Party. Ms. Marks, his campaign treasurer, previously admitted to having helped Mr. Santos falsify the records. She pleaded guilty to one felony count in October and faces up to four years in prison.

And Mr. Santos admitted to committing unemployment fraud during the coronavirus pandemic and lying on House financial disclosure documents to inflate his wealth.

Under the terms of the plea deal, Mr. Santos is required to forfeit over $200,000 that he obtained illegally — money that must be repaid 30 days before his sentencing.

But Mr. Santos’s attorney, Joseph Murray, said that might not be possible. “At this time, he does not have the money,” Mr. Murray explained to Judge Joanna Seybert, promising that his client would make his “best efforts” to get it together before the deadline.

“He’s not saying he’s willfully going to default,” Mr. Murray said, adding that he merely wanted to advise the court that he might request more time “if for some reason he’s totally unemployable and can’t pay it.”