He went to bed a street sweeper. He woke up a national celebrity.

Then came the video.

Martínez, a 24-year-old aspiring musician and composer, surreptitiously recorded himself riding on the back of his garbage truck as it rumbled down the city’s tree-lined Reforma Avenue, his fellow sweepers unaware.

He made several takes early one morning and then laid the images over a plaintive, romantic song he’d written about a love scorned. At 7 p.m. on Jan. 27, he posted it on TikTok. Waking up the next day for his 6 a.m. shift, Martínez was shocked to see “Sueña Lindo, Corazón" (“Beautiful Dream, My Love") had around 100,000 views. His previous videos had topped out at about 1,000.

“That day I went to work, but I was really distracted because I kept checking my TikTok," Martínez said in an interview.

Two weeks later, he quit.

The song’s nostalgic tone struck a chord, and Martínez became an overnight sensation.

His TikTok video has been viewed more than 47 million times. His Spotify ballooned from a few thousand monthly listeners to around two million. People now recognize Martínez on the streets of Mexico City and want selfies with him. His phone number was leaked to the public, and fans sent him messages on WhatsApp. When he went to bid his co-sweepers adios, a circus of fans, TV crews and record-company employees erupted around him.

“I was trying to focus on my music, but I was filled with emotions," said Martínez, who admirers simply call Macario. “It was too much. I don’t know how I endured it and processed it."

Martínez’s rise from obscurity to fame has resonated in a relatively poor country with limited social mobility. He has emerged as a working-class folk hero.

“Achieve your dreams, Macario, for those of us who can’t shine like you," wrote one follower on TikTok. “If the rich have their artists for their parties, we have you for our struggles," another said.

It’s also a testimony to the ways in which TikTok has changed how musicians are discovered and gain fame, as viral videos bypass the record labels that were once the industry’s traditional gatekeepers. The platform has helped put new music and relatively unknown artists on the charts, including Olivia Rodrigo, Lil Nas X—and Martínez.

Born and raised in Mexico City, Martínez had harbored dreams of a musical career since he was in high school. After graduating, he worked as a gardener and then as a street sweeper to make ends meet, nurturing his music during his free time at night.

He had intended to go to college to study filmmaking, another one of his interests along with photography, but he failed Mexico’s competitive college entry exam twice as he cared for family members.

“Many of us Mexicans work in areas that we don’t dream about," Martínez said, “but we do it to keep ourselves afloat and to be able to work on the projects we really love."

The musician cried during a February TV interview as he heard “Sueña Lindo" being played live on air for the first time.

“I think his authentic reaction helped people connect with him," said Omar Pérez Reyes, a popular Mexican TV and radio personality known as “Faisy" who interviewed Martínez that day. “He showed his vulnerability."

That studio appearance with Pérez and a follow-up video of the TV host buying Martínez a new Godin guitar in a Mexico City music shop garnered tens of millions of views on social media, adding to the momentum.

Abraham Serey, a Mexican artist and social-media personality, published one of many videos about Martínez that sought to explain his rise to stardom. “Like many artists, his days were filled with challenges, but he had so much passion that he never gave up," Serey said.

A self-taught musician, Martínez began composing and performing in 2019, playing under a pedestrian bridge for tips and at local bars and pizzerias for friends while recording songs and videos in a makeshift studio and posting them on social media.

His music has its roots in the Gulf state of Veracruz, home to the “son jarocho" folk genre that uses a small double-stringed guitar and a donkey’s jaw for percussion. He also draws inspiration from dream pop, an alt-rock genre that uses effects such as reverb, echo, tremolo and chorus.

“Sueña Lindo" typifies those influences. Its hazy, atmospheric mix of guitar chords and ambient sounds matches the longing in the lyrics, about the pain of a breakup and how Martínez was able to come to terms with it.

“The song is about nurturing the dreams of a person whom you love even if that person isn’t ready to love you," he said. “It’s saying, ‘rest, sleep well, and don’t worry about anything because I will always love you no matter what.’ "

Now focusing on his music full-time, Martínez said he is retaining ownership of his songs but signed a contract with Sony Music Publishing to manage ​them, protect their copyright and to secure opportunities for their placement in media​ and brand deals.

Martínez has performed at major Mexican music festivals and collaborated with other artists, including the regional Mexican group Eslabon Armado and rapper Sabino. He is recording new singles, the latest set to be released on May 9.

He says he’s thrilled with his success and is learning to live with his fame.

His first performance after “Sueña Lindo" went viral was a free concert on Valentine’s Day in front of a few hundred enthusiastic fans at a landmark outdoor Mexico City cultural center.

Martínez invited his former co-workers to the front row, where they cheered him on.

Write to Robert P. Walzer at robert.walzer@wsj.com