On Opening Day, April 3, 2017, Harper hit a solo home run, struck out, and walked against the Miami Marlins. His home run was the fifth of his career in a season opener, the most by a player younger than 25. He set the MLB record for runs scored in the month of April with 32, surpassing Larry Walker’s 29 in 1997.

After being hit by a pitch by Cole Hamels in the first inning of a game against the Phillies on May 6, Harper eventually advanced to third, then stole home plate, becoming the first teenager to steal home plate since 1964. Hamels later admitted that he intentionally hit Harper and was suspended for five games by MLB. On May 14, Harper was 19 when he hit his first career Major League home run, connecting off of San Diego Padres pitcher Tim Stauffer. He was the youngest player to homer in the major leagues since Adrián Beltré did in 1998. He was named National League Rookie of the Month for May.

Harper attended Las Vegas High School in Las Vegas, Nevada. In May 2009, Sports Illustrated featured Harper with a cover story in which he was compared with LeBron James. After his sophomore year, he earned his General Educational Development (GED) in October 2009, making him eligible for the Major League Baseball (MLB) draft held in June 2010, in order to begin his professional baseball career earlier.

For the 2010 college season, 17-year-old Harper enrolled at the College of Southern Nevada of the Scenic West Athletic Conference (SWAC) in the National Junior College Athletic Association (NJCAA), where he played as a catcher. His older brother Bryan, who had been his teammate at Las Vegas High School, was one of the Southern Nevada Coyotes’ starting pitchers, and the brothers often worked as a battery. An advantage for Harper in his eventual transition to his professional career was that the SWAC, like MLB, uses wooden bats in conference play. In 66 games, he hit 31 home runs with 98 runs batted in (RBIs), hitting .443 with a .526 on-base percentage (OBP), and a .987 slugging percentage (SLG). His 31 home runs broke the school’s previous record of 12. He was named the 2010 SWAC Player of the Year.

At the signing deadline, Harper and the Nationals agreed to a five-year contract worth $9.9 million, including a $6.25 million signing bonus and eight semesters of college tuition. When asked about the signing, Nationals President Stan Kasten said, “The truth is, with a full minute to go, Mike and I both thought we were not going to have a deal.” Asked what changed in that final minute, Nationals General Manager Mike Rizzo replied, “It was both sides compromising and knowing that we were so close, it would be fruitless not to get a deal done.” On August 26, 2010, Harper was introduced by the Nationals. He said he chose to wear No. 34 because “I always loved Mickey Mantle, three and four equals seven.”

After batting .399 in spring training, Harper began his minor league career when the Nationals optioned him to the Hagerstown Suns of the Class-A South Atlantic League. In April 2011, after a slow start in the minor leagues, Harper visited optometrist Dr. Keith Smithson, who reportedly told him, “I don’t know how you ever hit before. You have some of the worst eyes I’ve ever seen.” In his first 20 games after receiving contact lenses, Harper hit .480, collecting seven home runs, 10 doubles and 23 RBIs.

Harper was selected to represent the United States in the 2011 All-Star Futures Game during the 2011 All-Star Game weekend. He was promoted to the Harrisburg Senators of the Class AA Eastern League on July 4. Harper went 2-for-3 in his AA debut with two singles, a run, and a walk.

On August 18, 2011, Harper injured his hamstring while running from second to third on an extra-base hit. The injury was severe enough for him to be carried off the field by his coaches. He was placed on the seven-day disabled list and the injury had ended Harper’s season. Harper began the 2012 season with the Syracuse Chiefs of the Class AAA International League.

The Nationals promoted Harper to the major leagues on April 27, 2012, after Ryan Zimmerman was placed on the disabled list. He made his MLB debut with the Nationals the next day against the Los Angeles Dodgers. Harper grounded out to pitcher Chad Billingsley in his first major league at-bat. He recorded his first major league hit, a double, in his third at-bat against Billingsley and got his first RBI on a sacrifice fly in the top of the ninth against Javy Guerra.

After hitting 13 home runs in just 58 games, Harper was selected to participate in the 2013 Home Run Derby. Harper hit a total of 16 home runs in the first two rounds to advance to the final round, in which he faced Yoenis Céspedes, an outfielder for the Oakland Athletics. Although he lost 9–8 in the finals, Harper was the second-youngest player to participate in the Home Run Derby, and the youngest to ever make it to the final round. Harper hit his 17th homer of the season on August 6, the 39th of his career, passing Ken Griffey Jr. for most home runs by a player younger than 21. Only two other players hit more home runs than Harper before turning 21. In 118 games, he hit .274/.368/.486 with 20 HR, 58 RBIs, and 47 extra base hits. During the 2013 off-season, Harper successfully underwent left knee surgery to remove a bursa sac.

During a game against the San Diego Padres on April 25, 2014, Harper suffered a left thumb injury when he slid into third base on a 3-run triple. He was placed on the 15-day disabled list the next day. An MRI revealed that the thumb had a torn ulnar collateral ligament. On April 28, it was announced that Harper would require surgery to repair the ligament tear in the thumb. During a rehab game with the Double-A Harrisburg Senators on June 28, Harper hit 3 home runs in a single game. Harper returned to the Majors on June 30. In 100 games during the season, Harper batted .273 with 13 home runs and 32 RBI.

After the 2014 season, Harper planned to travel to Japan to participate in the 2014 Major League Baseball Japan All-Star Series, but later withdrew.

On May 29, in a game against the San Francisco Giants, Harper was hit by a pitch from Hunter Strickland. Harper had hit two home runs off of Strickland in the 2014 National League Division Series (a series the Giants won), leading Harper to believe Strickland was seeking revenge. Harper slammed down his bat and charged the mound, throwing his helmet wide of Strickland before the two players exchanged punches, starting a bench-clearing brawl in which Giants first baseman Michael Morse, one of several former Nationals teammates of Harper on the Giants team, suffered a serious concussion in a collision with Giants pitcher Jeff Samardzija. Both Harper and Strickland were ejected for their roles in the brawl. The next day, on May 30, Harper was suspended four games. Harper appealed, and his suspension was reduced to three games. Harper credited Morse with preventing him from injury during the scrum, as Morse had absorbed a body blow from Samardzija that appeared to be aimed at Harper, telling a MASN reporter, “I’m very thankful for Mikey Mo.”

Harper is a member of The Church of Jesus Christ of Latter-day Saints. He has spoken publicly about his decision to abstain from alcohol. In 2014, he said that he sometimes drinks coffee during the baseball season. Though serving a mission is strongly encouraged for male members of The Church of Jesus Christ of Latter-day Saints who are between the ages of 18 and 26, it is not required. Harper decided his mission rested on the diamond. In a 2016 interview, he said, “Coming up to the draft and trying to make that decision, I always thought that my Heavenly Father upstairs always just wanted me to be a walking Book of Mormon, you could say,” […] “I knew that I could touch a lot of people’s lives playing and trying to be the best Mormon that I can be on and off the field.”

Harper and his girlfriend Kayla Varner were engaged in 2014, but their wedding that was set for January 2015 did not take place. In July 2016, Kayla announced the couple’s reconciliation and re-engagement. Harper married his longtime girlfriend in a ceremony at the San Diego California Temple in December 2016. Their first child, a son, was born in Las Vegas on August 22, 2019, while Harper was with his wife on paternity leave.

On April 18, 2015, Harper hit the longest home run of his MLB career with a 461-foot drive over the center field wall against the Philadelphia Phillies. On May 6, Harper hit three home runs in a single game for the first time in his career. He did it against Tom Koehler, with one of the shots going 442 feet to the second deck in a 7–5 victory over the Miami Marlins. He became the youngest player to accomplish this feat since Joe Lahoud in 1969. Harper was later awarded the Player of the Month Award for May for the first time in his career.

Harper was featured in an episode of ESPN E:60. He was featured in another ESPN magazine in 2015, this time displaying naked in ESPN’s The Magazine 2015 Body Issue. Harper also received a sponsorship deal with a nutritional supplement company that focused on active lifestyles, MusclePharm. He also endorsed the barbershop chain and hair product line Blind Barber.

On April 14, 2016, Harper hit his first career grand slam for his 100th career home run in a game against the Atlanta Braves. Three days later, he hit a home run for a fourth straight game, setting a new career streak for most consecutive games with a home run. He was named National League Player of the Week on April 18, after driving in 12 runs, tying a club record for home runs in consecutive games and becoming the eighth-youngest player in major league history to reach 100 home runs.

The Nationals finished the season with a 95–67 record, clinching the NL East division, but lost to the Dodgers in the 2016 NLDS.

Harper was the top overall vote-getter for the National League to start in the 2017 Major League Baseball All-Star Game and received the most votes of any one player overall. As the starting right fielder for the National League All-Stars, he went 1-for-1 with a walk and made a diving catch to rob American League catcher Salvador Pérez of a base hit. He wore cleats for the All-Star Game in honor of the late Miami Marlins ace José Fernández, a division rival pitcher who had held Harper to a career .211 batting average against him; Fernández had been killed in a September 2016 boat crash.

On May 13, Harper and the Nationals avoided arbitration in 2018 by agreeing to a one-year, $21.625-million contract. That night, he hit a walk-off home run versus the Philadelphia Phillies for the second time in the 2017 season, after doing it on April 16 as well.

Harper participated in the 2018 Home Run Derby. Harper’s father also participated as the pitcher for Harper’s at-bats. Harper won the derby with nine home runs hit in 47 seconds.

In 2018, the Washington Nationals Dream Foundation and the District of Columbia Department of Parks and Recreation renovated a baseball field in the Takoma neighborhood of Washington, D.C. and named it Bryce Harper Field.

Harper is a fan of the Vegas Golden Knights of the NHL. He dropped the puck before a game in their inaugural season and put the Golden Knights’ logo on his bat during their run to the Stanley Cup Finals in 2018. After his first son’s birth, the Vegas Golden Knights gifted him a personalized jersey.

On March 2, 2019, Harper signed a 13-year, $330 million contract with the Philadelphia Phillies. This deal set the record for the largest MLB contract until Mike Trout signed a 12-year, $430 million contract extension with the Los Angeles Angels. Harper chose to wear No. 3 with the club, as the number he wore with the Nationals (34) was being considered for retirement by the Phillies in honor of the late Roy Halladay. At his first press conference, Harper misspoke saying “I can’t wait to bring the title back to D.C.”

Harper’s first hit as a member of the Phillies was a 465-foot home run to the second deck of the right field bleachers at Citizens Bank Park on March 31, 2019 off Jesse Biddle of the Atlanta Braves. On August 15, 2019, Harper hit a walk-off grand slam against the Chicago Cubs. On September 3, 2019, Harper would bat his 100th RBI of the season for the second consecutive year and was the first Philadelphia Phillies player since Ryan Howard in 2011 to hit for 100 RBIs or more in a season. However the Phillies missed the playoffs, while Harper’s former team Washington won the World Series.

In March 2020, the Major League Baseball season was suspended indefinitely due to the COVID-19 pandemic. On May 15, Harper took to Instagram and suggested a potential 135-game season that would last from July through November. Major League Baseball eventually settled on a 60-game season that began on July 24. Harper hit .268/.420/.542 with 13 home runs and 33 RBIs and led the majors with 49 walks during the season.