For a few hours on Tuesday evening, David Zaslav seemed like the happiest man in the world.

“Look at this!” the 63-year-old media executive proclaimed, gesturing at the ice-blue piscine of the Hotel du Cap-Eden-Roc, the seaside resort perched above the French Riviera with a swimming pool immortalized by Slim Aarons.

Yachts bobbed in the distance; searchlights crossed the sky. Somewhere in the crowd, Leonardo DiCaprio was wearing vegan sneakers, and Lily-Rose Depp was smoking a Cuban cigar.

Not so long ago, as the boss of middlebrow Discovery Inc., Mr. Zaslav was known as a cable TV magnate who retailed reality fare like “My 600-lb Life” and “Dr. Pimple Popper.” On the night of the party, a year after Discovery gobbled up WarnerMedia in a blockbuster deal — and put Mr. Zaslav the in charge of HBO, CNN and the Warner Bros. movie studio — he was huddling with Scarlett Johansson and having dinner at a table with Martin Scorsese and Robert De Niro.

Officially, the event, a social highlight of the Cannes Film Festival, was a 100th anniversary party for Warner Bros. Unofficially, it was the A-listification of Hollywood’s newest mogul.

As a relative newcomer to this rarefied milieu, Mr. Zaslav had enlisted an expert guide — his co-host for the evening, Graydon Carter, who made Vanity Fair’s Oscar party into a glittering annual event during his 25-year run as the magazine’s editor. Now Air Mail, Mr. Carter’s upscale newsletter, was making its own Cannes debut.

“I love the motion picture business, and to be here tonight at Cannes …” Mr. Zaslav said, trailing off. At one point, an illustration of himself and Mr. Carter driving a vintage Mercedes along the Riviera was projected onto the surface of the famous pool. “Here we are, two best friends, at the Hotel du Cap,” he said. “I don’t know — it doesn’t get better!”

Mr. Zaslav’s troubles were thousands of miles away.

Wags on Twitter were mocking the decision to drop the “HBO” from the streaming platform HBO Max, a change that went into effect hours before the party began. CNN was facing a prolonged backlash for its town hall with former President Donald J. Trump, and the network’s ratings have occasionally dipped below those of the right-wing cable news station Newsmax.

Then there was the Hollywood writers’ strike. While other studio chiefs like Ted Sarandos of Netflix had canceled appearances and kept a low profile amid the labor strife, Mr. Zaslav forged ahead. On Sunday, before jetting to France, he gave a commencement address at Boston University, where he was booed and heckled (and then memed) by students who chanted, “Pay your writers!”

There were no picket lines in the south of France, in part because local police banned them. And the Mediterranean mood was more forgiving.

The playwright Jeremy O. Harris was among the guests at the Hotel du Cap. “I have a show with HBO,” he said, “so David is still technically my boss, even though we’re on strike.” He added that he had come to Cannes “as an actor” — he has a role in a film shown at the festival, “The Sweet East” — and he had “tried to treat all the work and activism I do around the writers’ strike with the same amount of fervor here.”

Like other guests, Mr. Harris said he was excited to snag one of the custom-printed ashtrays. “I’m continuing to exist in the world, while also knowing that my corner of it has a really dark thing happening,” he said.

As Mr. Zaslav bopped about in Loro Piana loafers, his co-host, Mr. Carter, watched from a remove. This was Mr. Zaslav’s second Cannes trip; Mr. Carter, 73, has been a regular here for roughly 25 years.

Air Mail has a continental flavor and a growing readership but is not yet a household name. So, late last year, Mr. Carter dusted off the Rolodex and gave a party for his venture at the Odeon restaurant in Lower Manhattan. When Mr. Zaslav approached him with the idea of hosting something on Oscar night, he countered with Cannes, where he had held an annual party for much of his Vanity Fair tenure. Adding to the intrigue: Vanity Fair, now led by the editor Radhika Jones, was planning its own Hotel du Cap bash. In an extremely haute monde sense, this was war.

On Tuesday morning at the hotel, Mr. Carter was girding for a last-minute meeting about the seating chart. Over espresso, he demurred on the notion of a rivalry with his old employer and was coy about what he’d heard about the Vanity Fair event a few days earlier. “I’m sure it was fine,” he said.

(Jeff Bezos and Mr. De Niro came to the Vanity Fair party on Saturday night, though the weather was poor. Miuccia Prada, whose namesake brand co-hosted the event, did not attend; a Prada spokeswoman said there was a scheduling conflict.)

Mr. Carter said he met Mr. Zaslav 25 years ago through mutual overlords: the Newhouse family, who are the owners of Condé Nast and also investors in Discovery Inc. (The family has a stake in Warner Bros. Discovery, too.) The men bonded in part over a shared love of Turner Classic Movies, a channel that is now part of Mr. Zaslav’s stable. And now Mr. Carter has gamely taken on the Henry Higgins role.

He helped broker Mr. Zaslav’s purchase of the Beverly Hills home of the late producer Robert Evans, coordinating with his ex-wife, Ali MacGraw (a friend). Mr. Carter said he and his longtime architect, Basil Walter, who was also at the party, are redesigning the Warner Bros. commissary in Burbank, Calif. (“I thought, this is Hollywood — it shouldn’t look like a Marriott,” Mr. Carter said.) The refurbished canteen will feature banquettes and murals reminiscent of Monkey Bar, Mr. Carter’s former Manhattan restaurant.

During the last writers’ strike, in 2008, Mr. Carter canceled Vanity Fair’s Oscar party. What about this year in Cannes? “It’s a celebration of what writers have done, so it’s not like this is antithetical to their aim,” he said, adding, “Writers aren’t paid as much as they should be.”

A few minutes later, Mr. Zaslav bounded up and gave Mr. Carter a bro hug.

“I’ve been working hard for this party,” Mr. Zaslav said with a grin, “calling Graydon every two days and asking how it’s going.”

A Baby and a Maltipoo

After alighting from luxury cars in the palm-shaded driveway of the Hotel du Cap, guests were greeted on Tuesday with hugs and handshakes from Mr. Zaslav and Mr. Carter — in nearly matching tan linen blazers. “Not intentional,” Mr. Carter said, a Pygmalion warily eyeing his pupil.

Inside, there were enough ashtrays to fill the “21” Club back when everybody smoked. Staff in black and white livery were quick to proffer lighters for those with cigarettes.

The actor John C. Reilly, in a hat and three-piece suit, took in the view. Eva Longoria and Oliver Stone crossed paths. Ms. Depp and Troye Sivan, whose HBO series “The Idol” premiered the night before, looked alert despite an after-party that went until dawn. The heiress Daphne Guinness glittered in a silver gown and her signature bicolor updo. Boy George was there in a very tall hat.

The Hotel du Cap overlaps with many of Mr. Carter’s interests: Old Hollywood, the Lost Generation and midcentury European glamour. It is the “large, proud, rose-colored hotel” in the opening lines of Fitzgerald’s “Tender is the Night” and a favored haunt of Picasso and Hemingway, Burton and Taylor, Delon and Bardot. Ms. Bardot’s stepson, the Swiss artist Rolf Sachs, who arrived in a sweeping cobalt frock, observed that the hotel had long been “part of the creation of a new bourgeoisie.”

“Graydon has the oomph to bring together a very mixed crowd,” he said.

Dinner was served on Bernardaud plates. Dessert was an extravagant candy bar called “Comme un Snickers.” Things really kicked off when a well-dressed baby arrived a few minutes past 11 p.m., its stroller carried down a curving staircase by a hotel porter. Another popular guest was Maxi, a 3-month-old caramel-colored Maltipoo, who like many of the attendees had flown in from New York.

Maxi, who was busy licking an ice cube, declined to comment, but his owner, the film producer and Venetian slipper purveyor Stuart Parr, marveled at the tablescapes. “This is made out of ceramic, not plastic,” he said, pointing out an Air Mail-branded match striker nestled between white orchids. “That tells you everything.”

Sting walked in a few minutes before midnight. Ms. Johansson arrived with her husband, Colin Jost, after the Cannes premiere of “Asteroid City,” the Wes Anderson film in which she has a key role. She chatted on the dance floor with Bryan Lourd, her agent and the co-chairman of C.A.A., and Joe Kahn, the executive editor of The New York Times. (Mr. Kahn sat with Mr. Carter and Mr. Walter at dinner.)

Mr. Zaslav may have been the co-host, but some of the guests were still getting to know him. Michael Barker, a co-president of Sony Pictures Classics, said he had met Mr. Zaslav for the first time that evening and appreciated his sense of film history. Boy George said he found Mr. Zaslav “very sweet.”

As at any industry event, some business got done. Mr. Zaslav, Mr. De Niro and Mr. DiCaprio were tossing around new titles for “Wise Guys,” a mob drama that Mr. Zaslav greenlighted after taking over Warner Bros. Boy George said he was open to any roles the Hollywood crowd might throw his way: “I’ve been talking to Rebel Wilson about playing her gay friend.”

By 2 a.m., after the disco and new wave music faded and the house lights went up, Mr. Zaslav was still working the room. He shook hands with the D.J.

The view from the slowly emptying pool deck — purple sky, dark sea, white yachts — was more or less the same one that Jack Warner, the studio’s longtime president, once enjoyed from Villa Aujourd’hui, his summer home, about 700 meters away.

Vanessa Friedman contributed reporting.