LOS ANGELES — Nearly a decade ago, thousands of young undocumented immigrants packed school gyms, churches and community centers around the country to learn about a new program that would allow them to live openly, and to work legally, without fear of deportation. To apply, they would have to come out of the shadows and trust the government with personal information that could potentially expose their parents.

Hundreds of thousands of them took the risk, and enrolled in a program known as Deferred Action for Childhood Arrivals, or DACA, starting in 2012. Eligible for Social Security cards, driver’s licenses and financial aid for college, they enrolled in college, applied for jobs, got married and bought homes.

They called themselves “Dreamers,” though the future of their American dream was suddenly called into question nearly three years ago when President Trump, as part of his campaign to rein in illegal immigration, announced that the DACA protections would come to an end.

The immediate threat that ensued for the more than 825,000 people who were granted DACA status over the years was eased on Thursday, when the Supreme Court in a 5-4 decision ruled that the Trump administration did not offer sufficient justification to terminate the program.

There was celebration Thursday in front of the Supreme Court in Washington and among young immigrants across the country, who had feared that the court would side with the government and clear the way for a speedy end to their legal lifeline. Now, it is almost sure to remain in place until after the November election — although its future after that is as uncertain as ever.

“It feels amazing,” said Vanessa Pumar, 31, who came to the United States from Venezuela at the age of 11, and who is now an immigration lawyer. “I have been holding my breath. It feels like I can finally breathe.”

The DACA case was among the most significant that the Supreme Court has considered since the appointment of Justice Brett Kavanaugh secured a conservative majority on the court, and the most consequential legal test yet of President Trump’s immigration agenda.

The decision not only paves the way for immigrants to continue renewing their protected status, it also opens the program to some 66,000 undocumented young people who had been excluded since the Trump administration began to wind the program down.

“I woke up and this was the first thing I heard. I’m actually still shaking,” said Joana Cabrera, who came to the United States from the Philippines at the age of 9. “I’m unbelievably happy, because I was expecting the worst.”

Ms. Cabrera, 24, is on a team researching the use of robots in Covid-19 testing at the Chan Zuckerberg Biohub in San Francisco.

Leo Acevedo, who was brought across the border as a toddler by his mother, now an undocumented housekeeper in Los Angeles, said his mother broke the news to him Thursday morning that the court had ruled in his favor.

“I was pleasantly surprised that it was upheld,” said Mr. Acevedo, 23, who is studying architecture and works at a Los Angeles firm. “It means that I can continue to live in the U.S. worry-free, without being deported. It allows me to continue to build the life I have been building here.”

The program has been life-changing for more than just direct participants: Some 250,000 U.S.-born children have at least one parent who is a DACA beneficiary. All told, about 1.5 million people in the United States live with someone who is protected from deportation solely because of the program.

The story of how the “Dreamers” gained temporary legal status has been one of the few success stories for immigrants in recent years as a succession of new policies under the Trump administration has made it increasingly difficult to win asylum or apply for legal visas.

In early 2012, Ms. Pumar and several activists in Los Angeles huddled over several months, often late into the night, to research legal remedies for the hundreds of thousands of young people who had been brought to the country as children by undocumented parents. On the East Coast, other Dreamers met with White House officials to make their case.

Despite broad support among the general public for giving them a path to legalization, they argued, Congress had failed to agree on a fix, again and again.

“We had lived here most of our lives and we wanted to come out of the shadows,” said Ms. Pumar, who grew up in California. “We felt we belonged here and had a lot to contribute.”

On June 15, former President Obama stepped into the Rose Garden and announced the DACA program, intended as a temporary measure to protect what politicians from both parties have often seen as one of the most sympathetic classes of immigrants. About three-quarters of Americans favor granting DACA recipients legal status, according to a survey released on June 17 by the Pew Research Center.

To qualify for DACA protection, immigrants would have to have lived in the United States for much of their lives and meet age and other requirements, including passing a background check and completing high school or enlisting in the military.

The program grew swiftly, with about 650,000 undocumented young adults currently enrolled. The average DACA recipient is 25 years old, and the oldest is 38.

Many of the “Dreamers” have been building promising futures. In 2014, Ms. Pumar became the first DACA recipient to pass the California bar, and, thanks to the program’s work provisions, she was able to start working as an immigration lawyer at a legal-aid organization. Her older brother, Marco, 36, also a DACA recipient, started a company that employs 25 Americans in Orange County.

Then Donald Trump entered the White House and his first attorney general rescinded the program, thrusting the young adults into a state of prolonged uncertainty. Would they lose their jobs? Face deportation? Go back into the shadows like millions of undocumented immigrants, like their parents?

Almost immediately, the courts intervened. Judges ordered the government to keep accepting renewals for DACA status until the Supreme Court had decided whether Mr. Trump had the legal authority to terminate it. But no new applications were accepted.

Many people have not renewed since Mr. Trump terminated the program, weighing the cost against the likelihood that the protection might end, or concerned that the administration could use updated addresses to find their parents and deport them.

Monica Sandoval, 28, an emergency room nurse in Columbus, Ind., has lived in constant fear over the past several years that she would lose her job and have to give up her goal of pursuing a master’s degree to become a nurse practitioner.

“I don’t see myself doing anything else,” said Ms. Sandoval, who lately has been fighting to stabilize coronavirus patients on respirators. “If DACA ends for good, I have no idea how I will manage.”

Marisol Montejano, 36, just graduated with a math degree from California State University San Bernardino, and plans to become a high school teacher. Nearly a decade ago, though, the single mother of two had dropped out of college because she could not afford tuition and books. After obtaining DACA, she was able to return to school, working two jobs with her new legal working status.

Thursday’s decision was a big relief, she said: “I feel like I could breathe. I feel like I could tell my kids it’s going to be O.K.”

After it was announced, she said: “I sent messages to my professors, and I am, like, ‘I am still here I am not going to go anywhere.’ I feel so good.”

Yet, as Ms. Montejano sees it, it is not really over at all, not until Congress adopts a lasting solution for some of the country’s most vulnerable immigrants.

“I believe that we worked really hard; they should give us something permanent,” she said. “They should give us our residency or citizenship.”