He decided to strike out on his own as a producer in the late 1990s, forming a studio with the producer David Janollari — with financial backing from 20th Century Fox, thanks to Mr. Chernin. In that role, he squired a few network shows and had a critically acclaimed hit with “Six Feet Under,” Alan Ball’s darkly comic funeral-home series. It lasted five seasons on HBO and won the Golden Globe for best television drama in 2002.

“I felt the HBO glow way back then,” Mr. Greenblatt said.

He rejoined the executive ranks at Showtime, where Mr. Chernin had worked in the 1980s. “When I was at Showtime we were, of course, in the shadow of HBO,” Mr. Greenblatt said.

At the time, HBO was known for gritty, male antiheroes like Tony Soprano. Showtime went in a slightly different direction under Mr. Greenblatt, programming shows like “Weeds” with Mary Louise-Parker and “Nurse Jackie” with Edie Falco. A personal favorite for Mr. Greenblatt was “Dexter,” starring the former “Six Feet Under” star Michael C. Hall. Showtime became a serious home for original programming on his watch, with the number of subscribers rising to 19 million, a 52 percent jump over his tenure there.

In 2010, as NBC was trying to recover from the decision to replace its 10 p.m. dramas with a nightly Jay Leno talk show, Comcast was closing on its purchase of NBC Universal. When Comcast executives were looking for a new entertainment head, Mr. Chernin suggested his protégé.

Mr. Greenblatt orchestrated a turnaround at NBC, taking it from last place to first among viewers 18 to 49 years old, who are prized by advertisers. At the network, he followed the strategy that he has relied on throughout his career: find a difference-maker. At Fox, it was programming teen-friendly soaps when no one else was doing them. At Showtime, it was a new twist on the antihero. At NBC, it was “The Voice,” the uplifting hug-in of a show that struck a new note in a genre dominated by the acerbic “American Idol.” And it was not such a big jump from the heartwarming qualities of “The Voice” to the unapologetically emotional NBC hit “This Is Us.”

Mr. Greenblatt also made live musicals an unexpected force on broadcast television, with NBC’s productions of “The Sound of Music,” “The Wiz,” “Hairspray” and “Jesus Christ Superstar,” a winner of five Emmys. And he showed off his musical chops at an NBC presentation for advertisers, accompanying Ms. Parton on piano for a performance of her song “I Will Always Love You.”

When Mr. Greenblatt left the network last September, he had his piano — an 1898 Steinway grand, with ivory keys, which he bought at age 20 — moved from his NBC office to his home. But he wasn’t through with show business, saying at the time that he would love to run a company. Once again, his mentor stepped in.