Harry Thomas Jr., then a newly elected council member, hadn’t even taken office when he hatched a scam that eventually fleeced the D.C. government of more than $350,000 intended to help city children.

On Friday, after months of denying any wrongdoing, Thomas (D) pleaded guilty to crimes that will likely send him to prison, closing a chapter on the promising political career of the 51-year-old scion of a prominent D.C. family. “Guilty as charged,” Thomas said simply when asked for his plea, just 15 hours after having resigned his Ward 5 council seat, the same one once held by his father.

The allegations were laid out in lengthy court papers that revealed Thomas wasted no time in seeking to steal city funds for expensive cars, trips, clothes and meals. Within months of taking office in 2007, he had pocketed $25,000 in his first kickbacks.

During the next three years, federal prosecutors said, Thomas used taxpayer money as a “personal piggy bank,” embezzling $353,500 from April 2007 through August 2009 to purchase items ranging from a luxury Audi Q7 SUV to a flashy Victory motorcycle.

His conduct “can only be described as a flagrant abuse of the public trust,” said Ronald C. Machen Jr., the District’s top federal prosecutor. “Time and time again, Mr. Thomas used for his personal gain taxpayer funds that were intended to benefit the city’s most important resource: its children.”

Machen declined to say whether others, including anyone who might have facilitated Thomas’s scheme, will be charged.

Thomas said little during his 45-minute plea hearing but assured U.S. District Judge John D. Bates that he understood the rights he was giving up by pleading guilty to two felonies: theft concerning programs receiving federal funds and filing a false tax return. Under federal guidelines, he would face 37 to 46 months in prison at his sentencing May 3. Thomas will remain free until then.

His attorneys are expected to argue for a more lenient sentence and are likely to trumpet Thomas’s ties to the community and public service.

The son of the late three-term city council member, Thomas was a star football player at Woodrow Wilson High School and went on to Bowie State University. A past president of the D.C. Young Democrats, Thomas was the vice president of public affairs for the Public Benefits Corp., the former D.C. General Hospital.

As part of the plea deal, Thomas agreed to resign from office. He also must pay restitution of $353,500 to the District government and work with the Internal Revenue Service on payments for $346,000 in income he admitted he did not declare on tax returns for 2007, 2008 and 2009. It is not clear how he would make such payments while serving time in federal prison.

The judge’s wood-paneled fourth-floor courtroom was packed Friday with spectators and reporters who watched silently as Thomas, wearing a blue suit and silver tie, pleaded guilty in a high-profile public corruption investigation, one of several now targeting elected D.C. officials.

His mother, Romaine Thomas, and wife, Diane Romo Thomas, sat in front of dozens of supporters. As the hearing neared an end, Thomas’s mother, a former public school principal, read from a Bible as she gently rocked in her seat.

After the hearing, Thomas told reporters that “I made some poor decisions and acted in ways I simply should not have” but declined to answer questions.

Thomas is the first member of the D.C. Council to resign under duress since the District’s home rule government was first seated in 1975. The probe has its roots in suspicions first leveled during Thomas’s 2010 reelection campaign by his Republican opponent about his fundraising for a nonprofit group he founded.

The District’s attorney general at the time, Peter Nickles, initiated a probe of the charity, called Team Thomas. Nickles’s successor, Irvin B. Nathan, outlined the graft allegations in a civil suit in June and referred the case to the U.S. attorney’s office for criminal charges.

At the time, Thomas said that he had done nothing wrong.

However, it soon became clear that federal authorities were aggressively pursuing Thomas. Federal agents raided his home last month.

According to a statement of offense signed by Thomas, the former council member began plotting his embezzlement scheme in January 2007. The first step involved targeting city funds that had been given to the D.C. Children and Youth Investment Trust Corp., a nonprofit, public-private partnership dedicated to developing programs to help children.

After taking office, Thomas asked the trust to provide a $60,000 grant to a for-profit group — identified in court papers only as “Organization #1” — that provided arts-oriented youth programs. In July 2007, an unidentified person in control of that group issued two checks totaling $25,000 to entities controlled by Thomas: Team Thomas, a nonprofit organization, and HLT Development, a for-profit corporation. Thomas’s groups provided no services in return for the grant award, prosecutors say.

The arrangement with Children and Youth Investment Trust was repeated on a larger scale the next year with a $392,000 grant intended for youth baseball programs — a scheme previously detailed in the D.C. attorney general’s civil lawsuit. Thomas arranged with a group affiliated with Langston Golf Course in Ward 5 to apply for the grant, then send $306,000 back to the two groups controlled by Thomas, prosecutors say. Thomas used the money to finance a lavish lifestyle that included the $69,000 Audi, trips, clothes and meals.

To support the grant award, Thomas directed a council staff member to prepare falsified documents and reports for the group, the Langston 21st Century Foundation, describing salaries, trips and events that did not exist or were not paid for from the grant money.

In December, the city revised its agreement with the Children and Youth Investment Trust to ensure more thorough reporting and monitoring of its grants.

The federal investigation also looked at Thomas’s use of city money to pay for a presidential inaugural ball held in the John A. Wilson Building in 2009.

The ball was organized by the D.C. Young Democrats, then headed by a Thomas staff member. Thomas, prosecutors said, “was intimately involved” in planning the event, which was not attended by President Obama or prominent national politicians.

After the ball, Thomas realized that there were insufficient funds to cover the event’s costs and and sought help from Children and Youth Investment Trust, which provided $110,000.

But the trust’s executive director, Millicent D. West, told Thomas that the money could not be sent directly to the Young Democrats, according to prosecutors. West advised Thomas’s staff to “change the name of the entity that would receive the funds,” the court papers said. Thomas did so, and the money was funneled through a Langston-affiliated group. Thomas ended up being reimbursed $7,500 for ball-related expenses from the Young Democrats.

Court papers did not identify West by name. West, who is now head of the city's Homeland Security and Emergency Management Agency, did not dispute the facts alleged by prosecutors. But she said she considered the inaugural event a proper use of public funds.

“I didn’t see it as a political event,” she said. “From what I understood, the money was being used for a celebration calling young people together to celebrate a historic moment.”

The Thomas case is one of three known federal investigations involving sitting District politicians. Authorities continue to probe Mayor Vincent C. Gray’s (D) 2010 campaign, including allegations of illegal cash payoffs, and the D.C. elections board referred D.C. Council Chairman Kwame R. Brown’s (D) 2008 at-large campaign for a federal review of unreported transactions and other irregularities.

There is no city law barring someone from running for office after a felony conviction.

It will be months before there is new representation for residents of Ward 5, which encompasses most of the Northeast Washington quadrant west of the Anacostia River.

Staff writers Tim Craig and Nikita Stewart contributed to this report.