A toothy smile, pure joy distilled to its simplest essence, radiated across Juan Soto’s face during his first home run trot as a major leaguer Monday evening. The teenager — 19 years 208 days young — pointed to the sky when he reached home plate, exchanged animated high-fives with three teammates and signaled to his parents, who had traveled from the Dominican Republic a day earlier to be in the crowd at Nationals Park.

Before Soto reached the Washington Nationals’ dugout, Michael A. Taylor took off Soto’s helmet and the crop-haired rookie, revealing an on-the-spot sense of humor, feigned a hair flip, imitating Bryce Harper. He took a curtain call at his amazed teammates’ urging. Fans, sensing the moment, roared for the young man whose legend had preceded him.

Soto, a kid playing like anything but one, couldn’t stop smiling through it all, not after cracking the first pitch he saw in his first at-bat of his first major league start. It was a 90-mph, belt-high fastball — the kind of pitch Soto, a fastball hunter, targets — and he smashed it 422 feet off the back of the visitors’ bullpen in left-center field in the second inning for his first career hit, a three-run shot to ignite the Nationals’ 10-2 blowout win over the San Diego Padres that ended Washington’s three-game losing streak. It was an opposite-field, lefty-on-lefty power display few have produced in a Nationals uniform. It was an instant glimpse of his remarkable potential, a sign that the hype just might be real.

“I didn’t think it was going to go out of the park,” Soto said in English, a language he has picked up over the past three years. “I just run hard, the same I do in the minor leagues. Then I heard [all the] noise, and I know it was gone.”

A day after becoming the first player born in 1998 to appear in a major league game, Soto, batting sixth and playing left field, became the first teenager to homer in the majors since Harper in September 2012. Three innings later, Harper, the franchise’s first 19-year-old prodigy, displayed his own opposite-field pop with a solo shot.

Soto later singled in the sixth inning and sprinted first-to-home on Taylor’s triple. He grounded out and lined out to right field in his other plate appearances, finishing the night 2 for 4. Mark Reynolds added two homers, and Trea Turner doubled twice as the Nationals’ offense compiled a double-digit run total for the first time since May 1.

The outburst was in support of Gio Gonzalez, who continued his quietly impressive 2018 season with seven innings of two-run ball. The left-hander allowed two hits, walked three, struck out five and threw 111 pitches. His ERA through 10 starts is 2.38. Two relievers finished the job, including left-hander Tim Collins, who tallied two strikeouts in a scoreless eighth inning after getting called up from Class AAA Syracuse. It was the 5-foot-7 left-hander’s first appearance in the majors since the 2014 World Series after undergoing two Tommy John surgeries.

“It was nice to get into the first game and get that out of the way,” Collins said. “It’s kind of like your debut.”

Monday wasn’t Soto’s debut; that event occurred Sunday against the Los Angeles Dodgers, when a nervous Soto struck out in a pinch-hit appearance.

“I was just saying, ‘Ohh, my god,’ ” Soto said. “I was just trying to breathe down and do my job.”

The experience made Monday easier. Soto knew he would start either Monday or Tuesday, and Nationals Manager Dave Martinez notified him via text message earlier Monday that Monday was the day. A few hours later, Soto was the first player Martinez encountered when he walked into the Nationals’ clubhouse.

“He shook my hand and gave me a hug,” Martinez recalled, “and said, ‘I’m ready.’ ”

The Nationals (25-21), fueled by desperation, decided he was ready over the weekend — after Howie Kendrick ruptured his Achilles’ tendon — and called the wunderkind up on Sunday.

The move completed one of the fastest rises through the minors in recent history. A month earlier, Soto was in low Class A, smashing South Atlantic League pitching staffs to a pulp. Monday, after three promotions in 27 days, he became the youngest player to start a game in the big leagues since 2012.

Martinez said he decided to start Soto in left field Monday because Padres left-hander Robbie Erlin has reverse splits; lefties have fared better against him than righties during his career. It didn’t hurt that Soto had posted a 1.308 on-base-plus-slugging percentage in 49 plate appearances against lefties in the minors this season.

Soto took batting practice wearing a batting helmet without the curly W logo on it, in Harper’s group. The two exchanged a one-armed bro hug before Soto, who dedicated his first couple of rounds of batting practice to hitting the ball the other way, crushed a ball to the red seats in left-center field and deposited the next in the second deck in right field.

The exhibition foreshadowed the scene at 7:29 p.m., a moment reminiscent of one authored by the Nationals’ original teenage power-hitting sensation. Nearly six years earlier to the day, on May 14, 2012, Harper belted his first career home run, a rocket to straightaway center field at Nationals Park. It also came against the Padres. It prompted a curtain call, too.

“It’s exciting,” Martinez said. “It’s uplifting. Here’s a young man getting a chance to play in the big leagues at an early age. It pumps everyone up.”

Harper has since developed into the franchise cornerstone, a pillar the Nationals have stood on during their rise to prominence. The Nationals envision a similar path for Soto. Monday was a start worthy of a whopping smile.