Jason Kilar, the head of WarnerMedia who came aboard with ambitious plans for the company’s stand-alone streaming service and wound up serving half his tenure effectively as a lame duck, announced on Tuesday that he was leaving the company.

The departure, which had been widely expected, comes days before WarnerMedia is set to complete a merger with Discovery Inc. and a new leadership team comes aboard.

“Leading this team has been the honor my lifetime,” Mr. Kilar wrote in a memo to staff. “My heart is so full, and I am beyond thankful to each of you. There is no better team on the planet, and I will savor every last step as I wander the lot in Burbank several more times this week, with this team on my mind always.”

Mr. Kilar was hired by John Stankey, the chief executive of AT&T, WarnerMedia’s parent company, in April 2020, just weeks into a global pandemic, and a month before HBO Max, the company’s streaming service, was set to debut.

Mr. Kilar, a founder of Hulu and a veteran of Amazon, was focused on getting the company’s streaming service onto steady ground. Months into his tenure, Mr. Kilar set about enacting a plan dubbed “Project Popcorn,” his most contentious decision. With the pandemic raging, Mr. Kilar announced in a December 2020 post on Medium that the Warner Bros. 2021 film slate would be released simultaneously in theaters and on HBO Max, a decision that blindsided most filmmakers and their representatives.

Streaming analysts applauded the decision, and HBO Max did have a strong year. HBO and HBO Max finished 2021 with more than 70 million global subscribers, beating internal expectations, according to AT&T.

But the decision landed like a thud in creative circles in Hollywood. Richard Lovett, co-chairman of the Creative Artists Agency, called it the “epitome of a self-interested corporate maneuver intended to benefit your company while wreaking havoc on the industry.” Filmmakers like Denis Villeneuve, Christopher Nolan and David Chase all criticized it.

Just as Mr. Kilar was enacting his vision, his bosses at AT&T were secretly planning their exit from the entertainment business. In February 2021, one month into Project Popcorn, Mr. Stankey began discussions with David Zaslav, the chief executive of Discovery, about a merger, a conversation that continued into March and April.

Mr. Stankey kept Mr. Kilar in the dark about the deal until days before it was announced in May. Mr. Zaslav will take over the combined Warner Bros. Discovery company once the merger is finalized.

Mr. Kilar also oversaw the February 2022 departure of Jeff Zucker, the head of CNN, an exit that made headlines for weeks. Mr. Zucker said he was leaving because he had a romantic relationship with a subordinate, Allison Gollust, and failed to disclose it to the company. Mr. Kilar offered few details and no alternate explanation, leaving much of CNN’s staff angry.

Two weeks later, Mr. Kilar told staff members that Mr. Zucker and Ms. Gollust had committed “violations of company policies,” including CNN’s news standards and practices, a charge they have both denied.