A high school prodigy and No. 1 draft pick, Harper has shown flashes of being among the most dynamic talents in the game's history. But in part because of injuries, he has failed to consistently meet the lofty expectations surrounding him. That, plus a personality that at times could be called volatile or arrogant, has contributed to him being a lightning rod among fans, opponents and even teammates.
Here's a look at the first seven years of Harper's career:
2018: Market watch
Projecting how many millions Harper would land as a free agent already had been a topic of conversation for some time, setting up the most anticipated walk year of the free-agency era. How high could he drive the price tag? Despite an early power surge, Harper struggled throughout the first half of the season, hitting just .214 at the All-Star break, as the Nationals failed to live up to preseason expectations.
Harper boosted his image with a crowd-pleasing win at the Home Run Derby at Nationals Park that might have lit a spark. He got his average up to .249 by season's end, and he played in a career-high 159 games, which was important for a guy whose durability has been an issue. The 34 home runs and career-high 100 RBIs look nice, but the .889 OPS, the third-best mark of his career, didn't scream $330 million.
"It's definitely crazy walking in today and knowing this could be my last game at Nats Park in my white jersey," Harper said prior to Washington's final home game. "It's definitely something that you're not sure how you're going to react, and not sure what it's going to be like, if that's after the game, or even before. I knew I wanted to get here and put the uni on right away and just cherish that moment, if it's going to be the last time or not."
Career trend in four words: Show him the money
2017: Bounce-back year
Coming off a disappointing 2016 campaign, Harper was in the midst of a great season in 2017 that was derailed by a knee injury in August. Harper missed a month and a half after hyperextending his left knee when his foot slipped on a slick base. At the time of the injury, Harper was hitting .326 with 29 home runs and a 1.034 OPS, second best in baseball.
Harper's knack for rubbing some people the wrong way was on display in May when he was part of a memorable brawl triggered by Giants reliever Hunter Strickland. Strickland plunked Harper in apparent retaliation for a pair of long home runs Harper hit (and admired) in the playoffs two and a half years earlier, and Harper responded by charging the mound with vigor and throwing his helmet in the general vicinity of Strickland. (If Harper was trying to hit Strickland, he missed badly.)
"Sometimes you just have to go and get him," said Harper, whose frustration with baseball's unwritten code of conduct regarding celebrations has been an ongoing sore spot.
Career trend in four words: That's more like it
2016: Something's not right
Coming off an MVP season, Harper was in position to become the undisputed face of baseball, but in the offseason he suggested the game needed a face-lift. In ESPN The Magazine's baseball preview cover story, Harper talked about bringing more style and personality to the game, raising some eyebrows by saying, "It's a tired sport because you can't express yourself." (Based on MLB's ad campaign this past postseason, Harper's message apparently struck a chord.)
On the field, Harper got off to a blazing start (again), but things came to a screeching halt in early May, when he walked 13 times in a three-game series against Cubs, including six times (three intentionally) in the Sunday finale. Coincidence or not, from May 9 -- the day after the Cubs series -- through the end of the regular season, a span of 116 games, Harper hit just .238 (135th out of 148 qualified batters) while posting a .395 slugging percentage (127th) and a .752 OPS (101st). For the year, his batting average dropped 87 points, his OPS fell 295 points and his WAR went from 9.9 to 1.6.
Sports Illustrated reported that Harper played through a shoulder injury for much of the season, although the team disputed the story. Harper never really addressed the matter directly, and his agent, Scott Boras, said only that Harper had an "issue he battled with."
Career trend in four words: Don't believe the hype?
2015: The gold standard
This was the season when Harper's enormous potential was on full display and that helped entice the Phillies to give him $330 million. Harper hit a career-high .330, led the majors in home runs (42), OBP (.460), slugging (.649) and OPS (1.109), and he was the youngest unanimous MVP ever. The only player with a higher OPS in his age-22 season or younger is Ted Williams, who is widely regarded as the greatest hitter who ever lived.
Despite all that, the Nationals failed to make the playoffs as the team unraveled down the stretch -- and Harper, naturally, was at the center of the meltdown's signature moment when he had a heated dugout exchange with teammate Jonathan Papelbon.
Tensions were high between the two from earlier in the week when Harper spoke out against Papelbon hitting Manny Machado with a pitch following a Machado home run (and again questioned baseball's age-old codes), saying, "It's pretty tired. ... I'll probably get drilled tomorrow." Four days later, Papelbon confronted Harper when he failed to run to first after hitting a routine fly to left field. Things escalated quickly, with Papelbon choking Harper and pushing him against the dugout wall before teammates broke it up. Papelbon admitted he was at fault -- he was suspended by the Nats for the rest of the season -- but somehow this sort of thing seems to find Harper.
Career trend in four words: What we've waited for
2014: Thumbs down
Harper's season never really got going, as he missed more than two months after having surgery on his left thumb, which he injured while sliding into third base on a bases-loaded triple on April 25. Limited to 100 games, Harper failed to make the All-Star team for the only time in his career.
On the bright side, Harper had his best postseason, hitting three home runs (two off Strickland, see above) in a four-game division series loss to the Giants.
Career trend in four words: Still stuck in neutral
2013: Signs of vulnerability
At age 20, Harper became the youngest player to start an All-Star Game since Ken Griffey Jr. in 1990, and the fourth-youngest position player ever to start the ASG. He also made the final of the Home Run Derby.
But Harper's aggressive style cost him in May as he injured his left knee running into the right-field wall at Dodger Stadium. After missing more than a month, Harper homered off Yovani Gallardo in his first at-bat off the disabled list, but the phenom was limited to 118 games for the season.
Career trend in four words: Bump in the road
2012: Teen wonder
Two years after being one of the most obvious No. 1 picks in MLB draft history, Harper was called up to the big leagues, making his major league debut April 28 at Dodger Stadium. Harper hit 22 home runs as a 19-year-old, tying for the second most by a teenager in MLB history, a mark matched by Nationals teammate Juan Soto in 2018. Not surprisingly, Harper was named NL Rookie of the Year.
The fanfare and fascination surrounding Harper was considerable. Eight games into his career, Cole Hamels hit him with a pitch essentially in an attempt to keep him in his place -- "It's just, 'Welcome to the big leagues,'" Hamels said -- and Harper answered by eventually scoring by stealing home.
In June, Harper delivered what remains perhaps his most memorable quote. A reporter in Toronto asked if the then-19-year-old would have a celebratory postgame beer while legally able to drink in Canada. Harper responded, "That's a clown question, bro," and the phrase immediately went viral on social media.
Career trend in four words: Starting with a bang