ST. PETERSBURG—Yevgeny Prigozhin, the Kremlin’s favorite restaurateur, won a lucrative government contract to deliver school lunches across Moscow in 2011. Parents were soon up in arms. Their children wouldn’t eat the food, saying it smelled rotten.

As the bad publicity mounted, Mr. Prigozhin’s company, Concord Catering, launched a counterattack, a former colleague said. He hired young men and women to overwhelm the internet with comments and blog posts praising the food and dismissing the parents’ protests.

“In five minutes, pages were drowning in comments,” said Andrei Ilin, whose website serves as a discussion board about public schools. “And all the trolls were supporting Concord.”

On Friday, a federal grand jury empaneled by special counsel Robert Mueller indicted Mr. Prigozhin, some of his companies and a dozen of his employees on charges of conspiracy to defraud the U.S. by tampering in the 2016 presidential election.

The online army accused of sowing discord among American voters during the 2016 presidential campaign emerged from a tiny corner of the business empire built by Mr. Prigozhin, a millionaire who also operates some of St. Petersburg’s most prestigious restaurants, according to more than a dozen former colleagues, employees and others familiar with the operations.