TOKYO — The finish line of the 800 meters is not a place where runners make decisions. They gulp for air. They writhe. They operate with the primal parts of the brain designed for survival.

When Athing Mu arrives there, it looks as if she has run a different race. She’s an elegant pillar among carnage. Other runners roll on the track like extras in a war movie. Mu stands there, hands on hips, with all the strain of an office drone on a cigarette break. She must make a decision.

“Ever since a couple years ago when I started to get on TV, I literally don’t know what to do when I finish the race,” Mu said. “They keep the camera on you, and I’m like: ‘I’m trying to leave. What do I do? I’m done running.’ So, I mean, I try to stand a little bit, look around. I don’t know what I’m looking at. After a few seconds, just leave.”

Tuesday night at National Stadium, Mu arrived. At 19 years old, not quite three months separated from her freshman year at Texas A&M, Mu announced herself as one of the great runners in the world, a track and field superstar no longer in the making but in full. A daughter of Sudanese immigrants from Trenton, N.J., Mu obliterated the field and won gold in the women’s 800 meters in 1 minute 55.21 seconds, breaking Ajeé Wilson’s American record by 0.4 seconds.

There was never a doubt. Mu bolted to the front and held the race in her hands for the entirety. She turned what is supposed to be a tactical, grueling race into a personal showcase of everything she will give to track and field for the next decade or so: her perfect stride, her megawatt smile, her absolute power and grace.

“Athing just really made an intentioned announcement to the world,” said Raevyn Rogers, who won the bronze in 1:56.81.

Mu ran in gold-painted fingernails and a red barrette stamped with the word “CONFIDENT.” She said afterward she “most definitely” intends to become the first woman to win Olympic gold at 800 and 400 meters — and just the second runner after Cuban Alberto Juantorena.

“Because I want to do it,” Mu said. “We’re also going to break the 800 world record.”

Mu’s words dripped with practicality and zero boastfulness. Her career has taught her, and everyone else, to expect greatness. After Gabby Thomas followed Mu by winning bronze in the 200, she mentioned she hadn’t seen Mu race. Someone informed her Mu had won gold.

“I got that,” Thomas said. “I knew that was coming before I ever saw the results.”

Over the weekend, Mu had envisioned standing on the medal stand and crying. After she won, she realized the ceremony probably would not make her drop tears. She had merely done what she had expected.

“I’m not super shook or shocked or anything,” Mu said. “If the race didn’t go as planned, or something was to happen during the race, that would have probably been more shocking than winning the gold.”

Can it all really be so easy? After one heat, American 800 runner Clayton Murphy, the 2016 bronze medalist, texted Mu: “Quit toying with them.”

Mu’s parents moved from Sudan to the United States in 2000, two years before she was born, and settled in Trenton. The second-youngest of seven siblings, Mu started following her siblings — one of whom, Malual, would go on to run at Penn State — to the Trenton Track Club as a 5-year-old. Coach Al Jennings had taught a handful of Olympians, but by the time Mu turned 17, he believed he had never seen a better runner. Not at her age. Period.

Mu rearranged the youth record books. A month after turning 17, she made the 800 final at the U.S. championships. At the U.S. Olympic trials, she annihilated the field in a manner that made Secretariat at the Belmont a squeaker. The whole time, she radiated wholesome joy.

“I’m still the jolly, bubbly girl I’ve always been,” Mu said. “I’m pretty awkward.”

At her first Olympics, Mu never once felt nervous. From the gun, she grabbed the race by the throat. The 800 can be physical and filled with spills. Mu used her brilliance to eliminate the prospect of trouble.

“At the end of the day, I don’t want to leave anything up to chance,” Mu said. “I don’t want to get into a race and get mixed in there and mess up my chances of reaching any of my goals. If I want to win a gold medal, I’m not going to leave it up to anyone else to decide my race.”

By the end of the first lap, Mu had removed any doubt. She continued to widen the gap, loping in perfect posture. Britain’s Keely Hodgkinson won silver with ease and still became a speck on Mu’s rearview.

“Athing is an incredible athlete,” Thomas said. “She has so much grace, so much composure at such a young age. She’s an amazing teammate. She’s an inspiration to me.”

After she crossed the finish line, Mu high-fived Team USA staffers in the stands. She posed with Rogers, an American flag wrapped around each of their backs. Rogers had earned her medal with a wicked sprint in the final 30 meters, a move that left her depleted but ecstatic.

“It’s a huge accomplishment,” Rogers said. “I feel like I’m always hard on myself, I beat myself up a little bit. But this is something I’m really proud.”

Mu is likely to race again at these Olympics. At the NCAA championships in early June, Mu led Texas A&M to a record in the 4x400 relay. Mu could run that race again in Tokyo. Her second gold medal could double as Allyson Felix’s last.

Felix will retire after these Games. Another precocious New Jerseyan, Sydney McLaughlin, will vie for the title of the face of women’s track and field in America.

“I’m going to let my capabilities and myself as an athlete be known,” Mu said. “I’m not going to be the face of track and field, but I’m going to be the face of myself. I’m going to be who I am.”

On the other side of the globe, a swath of Americans had just discovered who Mu is. They will learn more about her over the years, as she collects medals and charms the world. For now, Mu was asked, what should those people know about her? She placed her finger on her lips.

“I’m very fun,” Mu said. “This isn’t the last time you’re going to see me run. There’s more.

“My time is now. That’s what they can take away from this event. Watching anything in the future, I’m going to maintain with that statement. My time is now. Six years from now. Two years from now. It’s going to be my time.”

This story has been updated.