BOSTON — The central figure in the college admissions cheating scandal is William Singer, 58, a Newport Beach, Calif., businessman who prosecutors say managed and profited from the cheating scheme.

Mr. Singer, sometimes known as Rick Singer, has been cooperating with federal investigators since September 2018. He pleaded guilty to four charges in federal court in Boston on Tuesday.

[Read more on the Justice Department’s largest ever college admissions prosecution]

A tan, lean-faced man with close-cut gray hair, Mr. Singer sat very still and looked intently at the judge, Rya W. Zobel, as she explained the charges and detailed the rights he was giving up by pleading guilty. He answered, “Yes, ma’am” whenever the judge asked him if he understood.

When the judge asked Mr. Singer to describe his role in the plot, he became expansive, speaking for about 10 minutes about his methods.