J. D. Martinez was born on August 21, 1987, as Julio Daniel "J. D." Martinez, the son of Julio and Mayra, of Cuban descent. He was raised along with his five sisters. Martinez completed his elementary education from Flanagan High School in 2006. Before joining Nova Southeastern University in Davie, he played high school baseball at Flanagan High.

Martinez was drafted by the Minnesota Twins, but he did not play with the Twins. During his amateur career, he became two-time All-Sunshine State Conference baseball player.

The Houston Astros selected Martinez in the 20th round of the 2009 Major League Baseball draft as the 611th overall pickup. He then began his professional career with Rookie League, The Greeneville Astros.

Martinez later earned a promotion to Tri-City Valley Cats after batting .403. Following this, he played for Lexington Legends and Corpus Christi Hooks. He was awarded the SAL Player of the Year, Most valuable and Houston Astros’ Minor League Player of the Year as well.

He was invited to replace Hunter Pence in July, 2011. He was named as the first player selected out of Nova Southeastern University to debut in MLB.

Martinez was signed by Detroit Tigers on March 24, 2014, to a minor league contract. The Tigers later purchased his contract after he hit 10 home runs and drove in 22 runs in his first 17 games. The same year he was named as the American League Player of the Week and player of the month in June 2014.

He got his big-break form his 2014 season as he finished the season with 30 doubles, 23 home runs, 76 RBIs, a .315 batting average, and a.912 OPS. He became the finalist for the Player Choice Award for AL Comeback Player of the year.

Martinez earned his first career Silver Slugger Award following his 2015b season as he finished the season with a 282 batting average and reached career highs of 33 doubles, 38 home runs, and 102 RBI.

He renewed his contract with the Tigers for next two-year, $18.5. However, he was traded to Arizona Diamondbacks for Jose King, Sergio Alcantara, and Dawel Lugo.

He became the first player in Arizona history to hit four home runs in one game. On September 17, 2017, he reached to his 40th home run against the San Francisco Giants. He was the fifth player who reached 40 home runs while playing for two different teams in one season. After Bryce Harper, he became the first NL player to win consecutive player of the week award.

Martinez signed a five-year contract with the Red Sox on February 26, 2018, for $110 million. On April 8t, Martinez hit his first home run in a Red Sox uniform at Fenway Park. He won the AL Player of the Week Award for the week ending on May 20th, after hitting five home runs.  At that point, he was tied with teammate Mookie Betts for the major league home run lead at 15, making them the first duo in Red Sox history to both hit at least 15 home runs within the first 50 games of the season. 

Batting .329 with 27 home runs and 73 RBIs, Martinez was named to the 2018 MLB All-Star Game as the American League's starting designated hitter. He won the AL Player of the Week Award, for the week ending August 12th, when he hit .464 with 11 RBIs and nine extra-base hits. He was also recognized as the AL Player of the Month for August, with a slash line of .373/.453/.686 with seven home runs and 25 RBIs in 26 games.

He led the Red Sox in home runs, RBIs, and hits. He bashed his 43rd homer of 2018 in the regular season finale at Fenway Park, and a career high 130 RBIs. He set the Red Sox franchise record for most homers by a player in his first season with the club. He passed Dick Stuart who belted 42 in his first season with Boston in 1963. He finished the 2018 season batting .330 with 43 HR and 130 RBI. The only other Red Sox ever to hit at least .330 with 40+ HR and 130+ RBI are Ted Williams (1949) and Jimmie Foxx (1936,1938). The last major leaguer to do that was Miguel Cabrera (2013). J.D. won the Silver Slugger Award given to the best hitter at each position.

Martinez languished on the free agent market for most of the off-season before accepting a $105 million deal the Sox had offered him at the end of 2018. All he did in 2019 was contend for the Triple Crown and establish himself as a legitimate superstar. He hit .304 with a team-leading 36 homers and a .939 OBP.

During the start-delayed 2020 season, J.D. was again Boston's primary designated hitter. He struggled at the plate, falling below the Mendoza Line(.200) on September 18th. Overall he batted .213 with seven home runs and 27 RBIs in 54 games.

J.D. returned in 2021 as Boston's primary designated hitter, starting the season batting 13-for-30 (.433) in Boston's first seven games. On April 11th, he recorded his third career game with three home runs, driving in four runs in a 14–9 win over the Orioles and was named AL Player of the Week. He was named to the American League roster for the All Star Game. In the ALCS Game #2, he hit a grandslam in the top of the first inning. He finished the season batting .286 with 28 HRs, and led the American League with 42 doubles.

Martinez's power declined at an alarming rate this season. Over 139 games he batted .274 with 16 home runs (the fewest he's hit in a season since 2013)