[6] He began playing baseball at age six and participated in international youth tournaments in Venezuela and the Dominican Republic.

[11] On July 31, 2014, the Astros traded Hernández, Jarred Cosart, and Austin Wates to the Miami Marlins for Jake Marisnick, Colin Moran, Francis Martes, and a compensatory draft pick.

[11] On December 10, 2014, Hernández was traded to the Los Angeles Dodgers along with Chris Hatcher, Austin Barnes, and Andrew Heaney, in exchange for Dan Haren, Dee Gordon, Miguel Rojas, and cash.

[11] In August, Hernández replaced a slumping Joc Pederson as the primary starting center fielder,[17] though he missed much of September with a hamstring strain.

[11] Hernández was eligible for salary arbitration for the first time after the season, and in January 2018 signed a one-year, $1.6 million contract for 2018.

[23] On July 25, 2018, Hernández pitched in a game for the first time in his career, entering in the 16th inning against the Philadelphia Phillies and picking up the loss after walking two batters and giving up a three-run walk-off home run.

[11] In the postseason, he was two for 12 in the NLDS against the Atlanta Braves, one for 14 in the NLCS against the Milwaukee Brewers and two for 15 in the 2018 World Series against the Boston Red Sox, with one home run.

[11] In Game 7 of the 2020 National League Championship Series, Hernandez hit a game-tying solo home run in the sixth inning.

With the homerun, Hernandez also became the first player to pinch hit a game-tying or go-ahead home run in a winner-take-all postseason game.

[37] Hernández missed several games from late August into early September due to being on the COVID-19 related injured list.

On October 11, Hernandez hit a sacrifice fly in Game 4 of the ALDS to send the Red Sox to the American League Championship Series.

[11] During the final three games of the Division Series and first two games of the League Championship Series, Hernández set a new MLB record for the most total bases in a five-game playoff span, with 34; it also set a new Red Sox franchise record for the most total bases in any five-game span.

[46] On September 6, the Red Sox announced that Hernández signed a one-year, $10 million contract extension for the 2023 season.

[11] Prior to the 2023 season, with longtime shortstop Xander Bogaerts gone, Hernández emerged as a leader in the Red Sox clubhouse.

[49] With both the departure of Bogaerts, and the injury of Trevor Story, Hernández was named to be the Red Sox starting shortstop come opening day.

[51] However, multiple injuries for the Red Sox soon made them move Hernández back to the position, as well as some starts at second base.

[11] On July 25, 2023, the Red Sox traded Hernández back to the Dodgers in exchange for Nick Robertson and Justin Hagenman.

[55][56] He played in 126 games in 2024, appearing at every position except catcher and right field, though getting most of his time at third base while Max Muncy was on the injured list.

[63] During an episode of the baseball podcast On Base with Mookie Betts, Hernández revealed that he has attention deficit hyperactivity disorder (ADHD), referring to it as both his "superpower" and "kryptonite" when asked about what attributes to his continued postseason success.