The industry in which Mr. Plepler thrived has been destabilized recently by the entry of big-spending digital rivals like Netflix and Amazon. Their threat to old ways of creating entertainment helped set in motion the recent mega-mergers in media and entertainment. In addition to AT&T’s purchase of Time Warner, the Walt Disney Company is acquiring much of 21st Century Fox.

Mr. Plepler joined HBO as a public relations official in 1992, some years after serving as an aide to Christopher J. Dodd, then a United States senator from Connecticut. Before long, Mr. Plepler oversaw HBO’s communications department, but his position was greater than his title suggested. He became the face of HBO’s New York operation, playing a key role in programming decisions.

He moved into the role of co-president in 2007, after the network’s chief executive at the time, Chris Albrecht, resigned after being accused of assaulting his girlfriend. Mr. Plepler became chief executive in 2013 and continued to show a knack for picking material that pleased critics while also delivering the occasional cheap thrill.

During the early stages of AT&T’s acquisition of Time Warner, Mr. Plepler strongly suggested that he would not give up his independence during an interview at his usual lunch spot, a corner booth at the Lambs Club, a restaurant two blocks from his office. In order for HBO to thrive, he insisted, there would have to be “a Chinese wall” between the network and its Dallas-based bosses. He also wondered aloud why the AT&T chief executive Randall Stephenson would buy HBO if he had any plan that would involve “messing with a winning game.”

Mr. Plepler’s future came into clearer focus last June, when AT&T completed its acquisition of Time Warner, which was also the corporate home of the Warner Bros. movie studio and the cable networks CNN, TBS and TNT. Mr. Stankey addressed roughly 150 HBO employees during a town-hall meeting at the cozy HBO Theater on the 15th floor of the company’s New York headquarters. He warned them of a “tough year” ahead, saying that it “will feel like childbirth.”

Mr. Plepler shared the stage with Mr. Stankey that day, and there was a moment of public tension between them after the AT&T veteran promised more investment in HBO and Mr. Plepler interjected, “Let’s give him a hand for that simple sentence!”

Reclaiming the stage, Mr. Stankey said, “We’ve got to make money at the end of the day, right?”

“We do that,” Mr. Plepler responded.