This summer’s breakout star is a 94-year-old actress

This common real-life scenario forms the premise for some fictional justice in the movie “Thelma," which opens in cinemas later this month. It is a comedy about a vigilante elder who, via mobility scooter, embarks on a quest to track down the scammers and reclaim her money.

She’s played by a woman on an extraordinary journey of her own: 94-year-old June Squibb. It is the first leading film role for the actress who’s been at it longer than most everybody.

“Having a career in the theater and film as long as I have, you understand grit and determination. I’ve always gone through my life like that, determined that I will do what I want to do," Squibb said in a video interview from her apartment in a Hawaiian-themed complex in the San Fernando Valley where she’s lived for the past 20 years.

Magnolia Pictures, an independent distributor that picked up “Thelma" at the Sundance Film Festival, will release it June 21 on more than 1,000 screens. That is the widest opening in the company’s two-decade history—the indie equivalent of a tentpole movie.

“As counter programming to a world on fire, we believe a wide swath of the country is ready to revel in the brilliance of June Squibb," said Magnolia co-heads Eamonn Bowles and Dori Begley in an email.

‘I knew what I was doing’

Squibb, a lifelong stage performer, didn’t break into movies and TV until her 60s. She was a character actor spicing up minor parts when, at age 84, she had a surprising star turn—an Oscar-nominated performance opposite Bruce Dern in 2013’s “Nebraska." In another big-screen moment for Squibb this month, Pixar’s “Inside Out 2," she’s the voice of Nostalgia.

Born in Illinois in 1929, Squibb started on stage in St. Louis and Cleveland before moving to New York to sing, dance and act. Aside from the occasional Christmas stint as a department store Santa’s helper, she said, “all my years, I made a living at it."

Amid off-Broadway musicals and comedy skits in cabaret clubs, in 1959 she made her Broadway debut in “Gypsy" alongside Ethel Merman. “But you sometimes go backwards," she said, recalling a stretch of low morale in her 30s while dancing at a Chinese restaurant. “I can remember thinking, why am I doing this? But I knew what I was doing. I was working."

Now she has top billing in a feisty comedy with a timely premise—elder fraud. Squibb’s character was modeled on an actual Thelma. The now 104-year-old grandmother of writer-director Josh Margolin became the target of a phone trick like the one in his movie.

Squibb’s version of Thelma draws motivation from a “Mission: Impossible" movie, taking cues from Tom Cruise as she sets out to reclaim her 10 grand. She takes to the streets on a scooter commandeered from her friend-turned-wingman Ben, played by “Shaft" great Richard Roundtree, who died of pancreatic cancer at 81 not long after making the movie.

Squibb, who stays fit with Pilates, executed many of her own (low-impact) stunts, such as driving the scooter and tumbling across a bed with a pistol in hand.

The younger characters, too, wrestle with age-related problems. Fred Hechinger (“The White Lotus") plays Thelma’s devoted 20-something grandson, who’s failing at adulthood and struggling to untether from his overbearing parents (Parker Posey and Clark Gregg), who also have the missing Thelma to fret about.

Margolin, a first-time director who also edited the film, relied heavily on Squibb’s comedic timing, like in a scene with Thelma running down a mental list of her late friends and the details of their passings.

“She has such an internal rhythm and knows how to deliver a line for a laugh and how to deliver a line to make you feel something," Margolin said. That dexterity helped the filmmaker set the story’s pace and tone “without tipping into parody."

Squibb credits “the harshest critic in the world"—her second husband Charles Kakatsakis, an acting teacher—for pushing her to develop the artistic tools and discipline for dramatic roles and work on camera. They’d been married 40 years when he died in 1999.

On television, she had a run on “The Young and the Restless" and became a master of the one- or two- episode appearance, popping up as a quirky neighbor or a meemaw or the voice of Michael Scott’s mom in “The Office." Weirder jobs were also welcome, like co-starring with Triumph the Insult Comic Dog on a short-lived Adult Swim series.

Jelly beans for energy

Squibb said she never aspired to be more than a useful character actor, much less a star. But her Academy Award nod—as the salty spouse of a man obsessed with a clearly bogus sweepstakes jackpot in Alexander Payne’s black-and-white “Nebraska"—put Squibb-ian zest in higher demand. She recently wrapped filming in New York on her next lead role, as the title character in Scarlett Johansson’s directorial debut, “Eleanor the Great," about a Florida senior who moves to New York solo after losing her best friend.

On set, when Squibb’s energy flags, she gets a pick-me-up from one of her preferred candies: jelly beans and caramels. “My assistant comes running over with one of the Werther’s. It helps!"

The actress, who turns 95 in November, said she still has more to explore in roles for her stage of life. “There is no reason why my work can’t continue to be good," Squibb said, “and maybe even very special."

Write to John Jurgensen at John.Jurgensen@wsj.com