Born and raised in Amory, Mississippi, Moreland attended Amory High School, where he competed in baseball and soccer. At Amory, he was a 2004 Louisville Slugger All-American after posting a 7–1 win-loss record with a 0.53 earned run average (ERA) as a senior, while also batting .456 and striking out just three times during the season. While in high school, he played on the same team as Derek Norris. After high school, Moreland attended Mississippi State University to play college baseball for the Mississippi State Bulldogs. There, he hit .343 with ten home runs his junior season and went 2–0 on the mound with a 3.46 ERA. He also appreciate a degree in general business administration. In 2006 and 2007, he played collegiate summer baseball with the Bourne Braves of the Cape Cod Baseball League.

Moreland played in two different levels of the Rangers’ organization in 2009. He started the season with the Class-A Advanced Bakersfield Blaze and hit .341 with 19 doubles, eight home runs and 26 RBIs. He was promoted to the Double-A Frisco RoughRiders where he batted .326 with 19 doubles, three triples, eight home runs and 59 RBIs. Moreland was assigned to the Arizona Fall League at the end of the season and played for the Surprise Rafters. He was named the “Tom Grieve Minor League Player of the Year” by the Texas Rangers after the season.

The Rangers invited Moreland to spring training in 2010. Moreland played with the Oklahoma City RedHawks (AAA) in the 2010 season until July 29. As of July 29, he had a season batting average of .289 with the RedHawks.

On July 29, 2010, Moreland was called up by the Rangers to take the roster spot of Ian Kinsler, who was put on the disabled list. Moreland had his first major league hit in his first at-bat, a single against Oakland Athletics pitcher Vin Mazzaro. Moreland finished the game 2-for-4, with two singles, as the Rangers won 7–4. On August 13, Moreland had the first home run of his major league career, off of Josh Beckett the Red Sox. On September 26, Moreland had his first multi-home run game of his major league career, hitting his sixth and seventh home runs of the year, in a 16–9 win over the Oakland Athletics. During the regular season, Moreland played in 47 games, batting .255 with nine home runs and 25 RBIs.

On June 21, 2011, Moreland hit his first career walk-off home run off of Enerio Del Rosario of the Houston Astros. For the 2011 season, Moreland batted .259 with 16 home runs and 51 RBIs in 134 games played. The Rangers were in the World Series for the second season in a row; Moreland had one home run during the Series against the St. Louis Cardinals, which the Rangers lost in seven games.

Moreland married Susannah Higgins on November 13, 2011. As of March 2019, the couple have three children; two sons and a daughter.

On June 9, 2012, Moreland became only the second American League batter—David Ortiz being the first—to hit a home run ball into AT&T Park’s McCovey Cove. Moreland’s 428-foot, 8th-inning blast off Giants pitcher Ryan Vogelsong was also the first pinch hit home run hit into the Cove by any non-Giants player. During the 2012 season, Moreland batted .275 with 15 home runs and 50 RBIs in 114 games played.

On May 6, 2014, Moreland pitched an inning against the Colorado Rockies in a 12–1 Rangers loss; he retired the side in order. Moreland did not play after June 7, due to an ankle injury that required surgery. In 52 games played, he had two home runs, 23 RBIs, and a .246 average.

The 2015 season was a career year for Moreland. In 132 games played, he was second on the Rangers behind Prince Fielder with a .278 batting average and 85 RBIs; he was tied with Fielder for the team lead with 23 home runs.

In 2016, Moreland batted .233, had 22 home runs and 60 RBIs in 147 games played. He also won his first Gold Glove Award.

On December 8, 2016, Moreland signed a one-year contract with the Boston Red Sox.

On August 25, 2017, Moreland made the second pitching appearance of his career, tossing an inning against the Baltimore Orioles in a 16–3 Red Sox loss; he allowed two hits but no runs, striking out one batter, catcher Caleb Joseph. During the regular season, Moreland played in 149 games, primarily as Boston’s first baseman, batting .246 with 22 home runs and 79 RBIs.

In the postseason, Moreland played in all four games of the ALDS, batting 5-for-13 (.385) as the Red Sox lost to the eventual 2017 World Series champions, the Houston Astros. On December 18, 2017, Moreland signed a two-year contract with the Red Sox.

Early in the 2018 season, Moreland split time with Hanley Ramírez as Boston’s first baseman, until Ramírez was designated for assignment on May 25. On July 8, Moreland was named to the 2018 MLB All-Star Game as an American League reserve first baseman. At that point in the season, Moreland had a slash line of .288/.358/.525 with 11 home runs and 43 RBIs in 71 games played. After the All Star break, Moreland struggled offensively, hitting .191 with 4 home runs in 47 games. Overall, Moreland finished hitting .245 with 15 home runs and 68 RBIs. In the postseason, Moreland appeared in 11 games, batting 5-for-17 (.294) as the Red Sox went on to win the World Series over the Los Angeles Dodgers.

On October 27, 2018, Moreland hit a three run home run, which put the Red Sox within one run of the Los Angeles Dodgers. The home run was viewed as a turning point in the series in which the Red Sox went on to win four games to one. In game four, the Red Sox won by a score of 9-6, and in game 5, they won 5-1 as the Red Sox went on to win their 9th championship in franchise history, and for Moreland it was his first.

Moreland and the Red Sox reached agreement on a one-year contract announced on January 28, 2020. With the 2020 Red Sox from the delayed start of the season through late August, he appeared in 22 games, batting .328 with eight home runs and 21 RBIs.

He was traded to the San Diego Padres on August 30, 2020, in exchange for minor leaguers Jeisson Rosario and Hudson Potts.