George Bratsenis shuffled into a New Jersey courtroom in shackles last month, an old-school hoodlum with thick glasses, white hair and a forearm tattoo.

Court is a familiar setting for Mr. Bratsenis, 73. So is prison. His Tarantino-esque odyssey spans at least three states and Canada and includes the killing of a drug courier known as the Turk, ties to organized crime and a bizarre failed jailbreak bid.

This court appearance was the result of his having been implicated in a murder-for-hire case involving a veteran campaign consultant that many in New Jersey’s political class think could grow into the state’s next big public corruption scandal.

The consultant, Sean Caddle, had admitted to arranging the killing of an associate who was found fatally stabbed in his burning Jersey City apartment in May 2014, and he had named Mr. Bratsenis as his hired hit man.

The February hearing ended abruptly without explanation, but on Thursday, Mr. Bratsenis pleaded guilty to taking part in the murder in exchange for thousands of dollars in cash from Mr. Caddle. He is scheduled to be sentenced on Aug. 2. Prosecutors are prepared to recommend a prison term of 10 to 25 years, his plea agreement says.

Appearing via video from a federal jail in Brooklyn, Mr. Bratsenis, wearing a gray T-shirt, answered, “Yes, Your Honor,” calmly and repeatedly as Judge John M. Vazquez of Federal District Court in Newark asked him whether he understood the implications of his plea. The only new detail to emerge involved the disclosure that Mr. Bratsenis agreed to plead guilty in the case last August.

“You all have a nice day,” he said as the hearing ended. “Take it easy.”

It is a new chapter for Mr. Bratsenis, whose criminal heyday lasted roughly from 1974 to 1985. In that time — according to interviews, court documents, other public records and newspaper articles — he racked up 10 convictions, many for felonies like burglary, armed robbery and, once, conspiracy to commit murder. He took orders from Gambino crime family associates and was an enforcer for a notorious former police lieutenant in Connecticut.

“He’s a very bad guy,” said Alan H. Nevas, who, as the U.S. attorney in Connecticut in the 1980s, won convictions against Mr. Bratsenis for bank robbery and in the murder conspiracy.

Mr. Bratsenis did not reply to an interview request sent to him at the Brooklyn jail, where he is awaiting sentencing in a separate bank robbery case. His lawyer declined to comment.

Born in Stamford, Conn., Mr. Bratsenis grew up with five sisters. (The one to whom he says he is closest did not answer a phone message.) In a court filing, he describes a childhood of walks to the park and Little League games.

He says in the same document that he began drinking as a teenager before moving to LSD, cocaine and heroin. He graduated from high school in 1966 — “enjoys girls, water skiing, baseball, hunting and cars,” his yearbook says — and then joined the Marines. He was honorably discharged, has been married and divorced twice — the first marriage lasted about a year — and has four children.

“I don’t want anything to do with him or his family,” his second wife, Patricia, said in a telephone interview. “I wish him nothing but yuck.”

Mr. Bratsenis’s criminal career began when he returned home from serving his country. His father had an exterminating business, but George chose a different path.

An early arrest came in October 1973, when he and three other men broke into a seafood restaurant in Westport, Conn. Several months later, he was arrested 800 miles away in Nova Scotia, Canada, on charges of robbing a bowling alley at gunpoint. He pleaded guilty to armed robbery and got 90 days in jail.

Back in Stamford, the Gambino and Genovese crime families were battling over control of the illegal rackets. Mr. Bratsenis and his cronies “would link up with organized crime associates and do their bidding — robberies, burglaries, selling drugs,” said Capt. Richard Conklin of the Stamford police.

“Subcontractors,” Michael Docimo, a retired Stamford officer, called them: “They’d do anything for hire.”

Mr. Bratsenis was “kind of a loose cannon” with a “certain meanness,” Captain Conklin said. At 6 foot 2 inches and over 200 pounds, he was also physically intimidating — a quality he used, Mr. Docimo said, as “muscle” for Larry Hogan, a retired Stamford police lieutenant and longtime target of law enforcement scrutiny over his suspected ties to the drug trade and organized crime. (Mr. Hogan was convicted in 1982 of trying to buy heroin from undercover federal agents. The verdict was reversed on appeal.)

Mr. Bratsenis’s primary partner was Louis Sclafani (he called himself Trigger Lou). From 1979 to 1983, court records show, they went on a whirlwind of crime: bank jobs in Connecticut; grand theft arrests in Florida; jewelry store heists and gun and drug convictions across New Jersey.

In 1980, the bullet-riddled body of David Avnayim, a.k.a. the Turk, was found in a car trunk near Mr. Hogan’s home. Four years later, Mr. Bratsenis, Mr. Sclafani and Mr. Hogan were charged in the killing. Mr. Hogan died before trial. Mr. Sclafani cooperated. Mr. Bratsenis pleaded guilty to a murder conspiracy charge.

David Golub, a lawyer who represented Mr. Hogan, said in an interview that he had read about the murder that Mr. Bratsenis was now implicated in.

“Is this about George Bratsenis rearing his ugly head?” Mr. Golub said when he answered the phone, adding: “If he’s out of jail more than a day, it’s a fluke.”

Mr. Bratsenis’s spree ended in 1983 when he was arrested and charged in one of the jewelry store robberies in New Jersey. Before his trial there, he was convicted of robbing two Stamford banks. (Mr. Sclafani cooperated again.) He got 30 years in federal prison for the robberies and 10 to 20 years, to be served concurrently, for the Turk’s murder.

Next came the New Jersey trial for the jewelry store thefts. Prosecutors said they followed a typical pattern: Mr. Bratsenis and his associates would scout places to rob. Then they would set fires elsewhere to distract the authorities.

The trial was a spectacle. Mr. Sclafani, testifying again as a federally protected witness, was brought to court under the watch of rooftop snipers guarding against a targeted killing.

Then there was Mr. Bratsenis’s escape plot.

While he was in prison awaiting trial, one of his sisters smuggled him a balloon filled with a nausea-inducing drug that he kept in his rectum for weeks. He planned to make himself ill by ingesting the drug the day the trial started, prompting a trip to a hospital. There, armed men would help him flee.

The scheme crumbled thanks to a jailhouse informer and an undercover agent who recorded Mr. Bratsenis’s sister discussing it. The trial proceeded, and he was convicted.

“Now, I’ve always said this in passing sentence, that it’s difficult to say whether an individual will commit another offense,” the judge said in ordering him to spend a minimum of 25 years in state prison. “But based on this defendant’s previous history, his attitude, there’s no question.”

Prescient words. Mr. Bratsenis wound up serving 25 years altogether — nine in federal prison and 16 in state custody — before being paroled in 2010.

Then, on Sept. 29, 2014, he was arrested in Trumbull, Conn., and charged in a bank robbery there. A second man, Bomani Africa, was also charged. Mr. Africa pleaded guilty and acknowledged that he and Mr. Bratsenis had stolen a car that they used in their getaway and then set ablaze.

The case got little notice until January, when Mr. Caddle, the campaign consultant, pleaded guilty to arranging the murder of Michael Galdieri, 52, and hiring Mr. Bratsenis. Mr. Africa pleaded guilty to taking part and said Mr. Bratsenis had recruited him.

Prosecutors say — and Mr. Africa, 61, has acknowledged — that he and Mr. Bratsenis met in a New Jersey prison where they were on the same cell block for several years and made plans to commit crimes together after being released.

Mr. Bratsenis’s link to Mr. Caddle is fuzzier. Mr. Caddle’s brother, James Caddle Jr., is one possibility. He, too, spent several years in the same prison as Mr. Bratsenis. He is dead, and it is not clear whether they met.

Another source of intrigue is the admission by Mr. Caddle, who controlled a web of dark money nonprofits and super PACs, that he has been cooperating with the F.B.I. in what his lawyer called an “important investigation.” It is unclear where his cooperation could lead.

The details of Mr. Galdieri’s murder prompted the son of a prominent New Jersey couple whose September 2014 deaths remain unsolved to seek an investigation into a possible connection between the cases.

In a letter to law enforcement authorities, the son, Mark Sheridan, wrote that, like Mr. Galdieri, his parents, John and Joyce Sheridan, who were found dead in their home, had been stabbed and that a fire had been set nearby.

No evidence linking the deaths has emerged, and the federal and state authorities have declined to comment.

Mr. Bratsenis, who has been in prison since being arrested in the bank robbery, is to be sentenced in that case next month — more than three years after his guilty plea. He has written to federal officials to complain about the lengthy wait and the conditions in which he has been held.

“At times, I was so upset I forgot where I was,” he wrote to a top prison official last March about the “inhumane” Metropolitan Correctional Center in Manhattan. “I said to myself: ‘Self, this is not the great U.S.A.’”

In a court filing, his lawyer, Charles Kurmay, sought a lenient sentence, citing Mr. Bratsenis’s prostate cancer and advanced age.

“He would like to be able to not die while in prison,” he wrote.