Craig Mazin was ready for a change.

About a decade ago, Mazin had carved out a solid career as a comedy screenwriter. Though his credits were hardly going to win over critics — “Scary Movie 3” and “Scary Movie 4” as well as the second and third installments of the “Hangover” trilogy, among them — the calls from Hollywood executives kept coming. It was a steady and lucrative job.

Still, there was something missing.

“A lot of what I would be offered was stuff where they’re like, ‘Who can fix this thing?’” Mazin recalled in an interview late last month. “Or ‘Can somebody get it from a C-plus to a B-minus?’”

Eventually he decided that “I’m better than the work I’m being offered,” he said.

That was the first crucial step in what would become a remarkable midcareer ascent. Over the last four years, Mazin, 51, has spawned two hit HBO series and transformed himself from a comedy screenwriter into one of the hottest showrunners in premium scripted television.

Mazin’s latest effort, “The Last of Us,” HBO’s adaptation of a video game revolving around an apocalypse, was an immediate hit. The network said that the first season of the show, which premiered in January, is averaging roughly 30 million viewers, a total in line with the “Game of Thrones” spinoff, “House of the Dragon,” and easily eclipsing the second seasons of other popular series like “Euphoria” (19.5 million) and “The White Lotus” (15.5 million). The first season finale will air on Sunday.

“The Last of Us” has also become the most successful adaptation of a video game into scripted entertainment — breaking a pitiful streak by Hollywood. While movies like “Warcraft” and the “Sonic the Hedgehog” films made plenty of money, few would consider them to be thoughtful storytelling. (Coming next month: “The Super Mario Bros. Movie.”)

For Mazin, it all started with that epiphany about nine years ago. At the time, he already had the respect of his peers — many of his screenwriter friends, including the “Game of Thrones” creators David Benioff and D.B. Weiss, had leaned on him for years for advice about their work. (Mazin has also been the co-host of “Scriptnotes,” a popular podcast that deconstructs the screenwriting process.)

“There was this enormous gap between how they saw me and how the business saw me,” he said.

He decided it was time to start listening to them and take a chance on himself, so Mazin set off to create his own project. He came upon a news article about the continued cleanup efforts at the Chernobyl nuclear reactor that was the site of the 1986 disaster. Mazin knew plenty about American catastrophes but little about Chernobyl, which is in Ukraine, at the time a part of the Soviet Union. He got to researching and was astonished by what he discovered. He devoured everything.

“The thing about Craig is, when he locks into something, he’s kind of obsessive,” said Casey Bloys, the chairman of HBO and HBO Max.

Still, pitching a dramatization of the fallout from a disaster three decades old was going to be an uphill climb. Mazin needed help. He was friendly with Carolyn Strauss, the esteemed former HBO programming executive who left the network in 2008 and began a career as a producer, with her first credit coming on “Game of Thrones.” Strauss knew how much Benioff and Weiss trusted Mazin professionally.

“He was a guy who they turned to for notes, for his structural mind, his story mind,” she said. “That whole crew respected his point of view on their work.”

Strauss joined the Chernobyl project as a producer and brought the idea to HBO, knowing full well that it would be a tough sell. Kary Antholis, who was then running HBO’s mini-series department, took the meeting because it was Strauss calling. And indeed, he had his doubts — both about Mazin’s unremarkable credits and the network’s willingness to invest in what was uniquely a Russian story. Then he heard Mazin’s pitch.

“It was the best pitch I’ve heard in 25 years of listening to pitches — there’s nothing that really comes close to it,” Antholis said.

Antholis convinced Sky to co-produce, lessening HBO’s financial burden. Expectations were low, and the series was given a Monday night time slot. Mazin said that he had been told repeatedly, “no one’s going to watch it.”

Instead, it was a hit with viewers and critics and a darling of the awards circuit. “Chernobyl,” which ran on HBO in 2019, won 10 Emmys and two Golden Globes, including the prize for best limited series from both.

This essentially gave Mazin carte blanche for whatever he wanted to do next at HBO. Mazin recalled that Bloys urged him to pursue whatever excited him most, asking, “What makes you levitate?”

Mazin was a dedicated gamer, going back to the late 1970s, when his father brought an Atari 2600 to their Staten Island home. When “The Last of Us” became a best-selling video game in 2013, Mazin bought a PlayStation console for it. He was mesmerized, particularly by the relationship between the two main characters: A tough, middle-aged survivor named Joel, and a 14-year-old girl named Ellie who is immune to the infection that turned most people on the planet into zombies.

A month after “Chernobyl” had wrapped up its run, Mazin met with the game’s creator, Neil Druckmann, and the two hit it off. They took the idea to HBO in July 2019.

“Casey, I found the thing that makes me levitate,” Mazin said he told Bloys. “Please, please, please buy this for me.”

Bloys did not play video games, and he was plenty familiar with Hollywood’s checkered history with adapting them. But there was a tradition at HBO in which writers who create a successful project are allowed to do whatever they want for their next one, no matter how different: Alan Ball went from “Six Feet Under” to “True Blood”; Mike White from “Enlightened” to “The White Lotus”; Michael Patrick King from “Sex and the City” to “The Comeback.”

“You’re making a bet on somebody, and it requires trust on both sides,” Bloys said. “It’s always going to be a leap of faith until you see it.”

“The Last of Us” was also an expensive gamble for the network, unlike “Chernobyl,” which had a budget of roughly $40 million, only $15 million of which came out of HBO’s programming budget, Antholis said.

“The Last of Us” was going to cost HBO more than $150 million, not far from “House of the Dragon.” It also had to fill HBO’s prime Sunday night slot, bridging the gap between “The White Lotus,” which ended in December, and the final season of “Succession,” which will premiere in late March. HBO and its debt-ridden parent company, Warner Bros. Discovery, desperately needed a supply of new hits to keep people from canceling HBO Max. The stakes were high.

First, the reviews came in: Critics were ecstatic, just as they had been with “Chernobyl” four years earlier. And then the ratings came in: It was an enormous hit.

The third episode boosted the show’s profile even further. Social media lit up with delight over the stand-alone installment, which was written by Mazin and mostly diverted from the source material: It centered on the marriage and survival of two peripheral characters, Bill and Frank, played by Nick Offerman and Murray Bartlett.

“What struck me about Bill and Frank and the potential for that story is to show a kind of love that we just don’t give a lot of attention to: The love between committed adults that are not getting younger,” Mazin said.

“The Last of Us” has already been renewed, and Mazin is weeks away from starting to write scripts for Season 2 with Mr. Druckmann.

In hindsight, the chance to do any of this all began once Mazin decided he was done with being pigeonholed by Hollywood executives.

“The industry doesn’t understand who people are — it only understands what they’ve written today,” he said. “One of the traps that you can get in as a comedy writer is their insistence that you must continue to only do that, because there just aren’t that many people that they can reliably hire to do those things. So they’ll keep you there.

“It was risky,” he continued, “but also exhilarating to just say, ‘I think I’m going to allow myself the freedom to do something else.’”