Sinclair pleaded to the lesser charges in exchange for the Army dropping sexual assault charges and two other counts that might have required him to register as a sex offender.
Sinclair, 51, had been accused of twice forcing a female captain under his command to perform oral sex during a three-year extramarital affair. The Associated Press does not generally identify alleged victims of sexual assault.
Defense attorney Richard Scheff said Monday that Sinclair is admitting to his mistakes, but added that the general is pleading guilty to behavior that likely wouldn't be criminal in the civilian world.
Scheff said he expected Sinclair to "to retire at a reduced rank and go home to his family." Scheff said he understands that the military needs to take a harder line against sexual assault but that there must be a balance: "It doesn't mean every complaint that's brought should go forward."
The Army's case against Sinclair started to crumble as questions arose about whether his primary accuser had lied in a pre-trial hearing. It was further thrown into jeopardy last week when Judge Col. James Pohl said the military may have improperly pressed ahead with the trial to send a message about its determination to curb rape and other widespread misconduct. Under the military code of justice, the decision was supposed to be decided solely on the evidence, not its broader political implications.
Ultimately, a judge will give Sinclair a sentence that can't exceed terms in the agreement struck between defense lawyers and military attorneys. The legal agreement is likely to require a punishment far less severe than the maximum penalties of 15 years in prison and dismissal from the Army.
Sinclair may face additional administrative penalties from the Army, which could force him to retire at reduced rank. That could cost Sinclair hundreds of thousands of dollars in pension benefits.
Retired Maj. Gen. Walt Huffman, a Texas Tech University law professor who previously served as the Army's top lawyer, said Sinclair could be busted back two ranks to lieutenant colonel because the affair at the heart of the case began before his most recent promotion.
Huffman also said it's possible the judge could sentence Sinclair to a punishment lower than what's called for in the plea agreement.
"If the judge determines he was a good soldier who served his country well other than his inability to control his zipper, then the judge might cut him a break," Huffman said. "But either way, his career in the Army is going to be over."
Sinclair's new plea agreement was approved and signed over the weekend by a high-ranking general overseeing the case, according to a copy provided by the defense team.
The married 27-year Army veteran pleaded guilty earlier this month to having improper relationships with three subordinate officers, including the female captain who accused him of assault. He also pleaded guilty to adultery, which is a crime in the military.
Under the plea deal reached over the weekend, Sinclair also admits to abusing a government credit card he used while traveling to visit his mistress.
Prosecutors did not comment on the deal or the case before Monday's hearing.
According to the defense, a separate agreement reached with Fort Bragg commander Maj. Gen. Clarence K.K. Chinn, who approved the plea deal, will dictate what punishments Sinclair will receive.
That part of the agreement will remain secret until after Pohl conducts the sentencing hearing. That process will include testimony from about 20 witnesses.