A private equity investor who paid more than $1.2 million to secure the admission of his three children to elite universities as purported athletic recruits was sentenced on Wednesday to 15 months in prison, the longest sentence yet in the nationwide college admissions bribery case, prosecutors said.

Prosecutors said the investor, John Wilson, 62, of Lynnfield, Mass., conspired to use fraud and bribery to ensure that his children would be admitted to the University of Southern California, Harvard University and Stanford University as Division I athletic recruits — even though prosecutors said they wouldn’t have qualified based on their athletic credentials.

In addition to the prison term, Mr. Wilson was ordered to serve two years of supervised release, complete 400 hours of community service and pay a $200,000 fine. Judge Nathaniel M. Gorton of U.S. District for the District of Massachusetts also ordered him to pay $88,546 in restitution to the Internal Revenue Service.

In October, Mr. Wilson was convicted by a federal jury in Boston of eight charges, including two counts of federal programs bribery and one count of filing a false tax return.

Mr. Wilson and another parent, Gamal Abdelaziz, a former casino executive from Las Vegas, had been the first people to stand trial in the federal investigation known as Operation Varsity Blues. Others charged in the case, such as the actresses Felicity Huffman and Lori Loughlin, chose to plead guilty rather than take their chances before a jury.

Last week, Judge Gorton sentenced Mr. Abdelaziz to one year and one day in prison, which at the time was the longest sentence handed down in the case. Prosecutors said Mr. Abdelaziz had agreed in 2017 to pay $300,000 to secure his daughter’s admission to U.S.C. as a fake basketball recruit, even though she had not played basketball for more than a year.

More than 50 parents, coaches and others have been charged in the case, which, prosecutors said, was engineered by a corrupt college consultant, William Singer, who has been cooperating with federal investigators since September 2018.

Prosecutors said Mr. Wilson agreed in 2013 to pay Mr. Singer $220,000 to secure his son’s admission to U.S.C. as a water polo recruit based on fabricated swim times and awards.

Mr. Wilson then deducted those payments from his tax return by claiming they were a donation to Mr. Singer’s sham charity, the Key Worldwide Foundation, and “business consulting” expenses, prosecutors said.

Although Mr. Wilson’s son did play water polo, he was not good enough to compete for U.S.C., prosecutors said.

In 2018, Mr. Wilson agreed to pay Mr. Singer $1.5 million to have his twin daughters admitted to Harvard and Stanford as athletic recruits, even though the daughters, who were good students, were not college-level athletes, prosecutors said.

Prosecutors had asked the court to sentence Mr. Wilson to 21 months in prison, writing in a sentencing memorandum that his “failure to accept responsibility remains strident, and his brazen disregard for the truth continues.”

“After overcoming what he describes as a difficult childhood to attend Harvard Business School and achieve success as a business executive, there was no compelling reason, and no excuse, for Wilson’s behavior,” prosecutors wrote. “Driven by arrogance, and motivated by self-aggrandizement, Wilson was willing to do whatever it took to get what he wanted.”

Mr. Wilson’s lawyers had recommended a six-month sentence, saying he “deeply regrets” taking part in what Mr. Singer called a “side door” to the college admissions process reserved for athletic recruits.

“He regrets the negative impact his actions have had on the level of trust and confidence that people across America have in the college admissions process, and on the system of higher education in general,” Mr. Wilson’s lawyers wrote in their sentencing memorandum.

The lawyers added that Mr. Wilson was “sorry for any impact his actions may have had on those without financial resources,” and that he “particularly regrets the tremendous pain and humiliation he has caused his family.”

Mr. Wilson’s lawyers argued that he should not receive a harsher sentence than “people who behaved worse.” They said that of the 31 parents sentenced in the case, about half had received sentences ranging from no prison time to six weeks. The average sentence was less than three months, they said.

More than 70 supporters wrote to the court, praising Mr. Wilson’s character and charitable donations. Two of the letters came from members of the Kennedy family, Kerry Kennedy and Edward M. Kennedy Jr., who know Mr. Wilson as a neighbor in Hyannis Port, Mass., where he owns a home next to the Kennedy family estate.

On Wednesday, Noel Francisco, a lawyer for Mr. Wilson, said he would appeal the conviction. He argued that Mr. Wilson’s case was “fundamentally different” from others in the federal investigation because his children were “well qualified” for admission based on their actual credentials.

His son was “a nationally competitive water polo player who actually participated on U.S.C.’s water polo team during his freshman year,” Mr. Fransisco said. “His daughters had perfect and near-perfect ACT scores.”

In addition, he said, Mr. Wilson’s money did not go to “personally enrich anyone at the school.”

“Making a donation to improve a qualified applicant’s chances of admission is a well-established process at colleges and universities across the country, and is still in use today,” Mr. Francisco said. “It is not a crime.”