Kapler attended William Howard Taft Charter High School in Woodland Hills, Los Angeles. He played shortstop, second and third base for the school’s baseball team, hitting .313 in his senior season, and graduated in 1993 at age 17. In his four seasons of high school baseball, he never hit a home run. During the summer, he batted .350 with 4 home runs and 30 runs batted in (RBIs) for the Woodland Hills East American Legion team.

Kapler attended Cal State-Fullerton in fall 1993 on scholarship for one semester, before transferring to Moorpark College in the Fall of 1994. He was named First Team All-Western State Conference after batting .337 with seven home runs and 52 RBIs. He was inducted into the Moorpark College Athletic Hall of Fame in 2008.

Kapler was the 57th-round draft pick (1,487th overall) of the Detroit Tigers in the 1995 Major League Baseball draft. He was signed by scout Dennis Lieberthal, father of former Phillie Mike Lieberthal, after being offered a $10,000 signing bonus. Playing 63 games for the Jamestown Jammers after he signed, he tied for second in the Class A- New York–Penn League in doubles (with 19), fifth in extra-base hits (27), and batted .288/.351/.453.

In 1996, with the Fayetteville Generals, Kapler led the Class A South Atlantic League in hits (157), doubles (45; 2nd in the minor leagues), extra-base hits (71) and total bases (280), was second in homers (26), RBIs (99) and slugging (.534), 5th in batting (.300), 7th in runs (81) and 10th in on-base percentage (.380). He made the South Atlantic League All-Star team. He then played for the West Oahu CaneFires in the Hawaiian Winter League, leading the league in home runs with 7.

In 1997, with the Lakeland Flying Tigers, Kapler led the Class A+ Florida State League in doubles (40) and total bases (262), tied for first in extra-base hits (65), was 2nd in games, 3rd in hits (153), tied for 3rd in home runs (19) and RBIs (87), 4th in slugging percentage (.505), and tied for 4th in runs (87) and sacrifice flies (10), while batting .295. He was named a Florida State League mid-season and post-season All-Star. He then played for the Honolulu Sharks in the Hawaiian Winter League.

In 1998, with the Jacksonville Suns, Kapler won the Class AA Southern League Most Valuable Player Award. He hit a league-high 28 home runs, and also led the league in hits (176; 8th-most in the minors), runs (113; 6th-most in the minors), doubles (47; 3rd-most in the minor leagues; breaking the old doubles record of 44), RBIs (146; most in the minors in 1998, and most ever in the Southern League), extra-base hits (81; a league record), total bases (319; a league record), and sacrifice flies (11). He was 3rd in the league in slugging percentage (.583), 4th in OPS (.976), 5th in batting average (.322), and tied for 8th in triples (6). His league record for RBIs broke the 1986 record of 132 set by Terry Steinbach. He played in both the Double-A and Southern League All-Star Games, and was recognized as the MVP of the Southern League All-Star Game. He was also named to the Southern League’s post-season All-Star team, and named a Baseball America First Team Minor League All Star. He was honored as Minor League Player of the Year by USA Today, Baseball Weekly, The Sporting News, and USA Today, and was named Tigers Minor League Player of the Year and Detroit’s No. 1 prospect by Baseball America.

Kapler made his Major League debut in 1998 at the age of 22. He became the first Tiger since the inception of the draft in 1969 to be selected as late as the 57th round, and reach the majors.

In 1999, he hit his first career home run on April 30 against Albie Lopez of the Tampa Bay Devil Rays. Kapler’s 10 home runs in his first 64 games was the fastest by a Tiger rookie since 1954, and was not surpassed until 2008. For the season, Kapler wound up hitting a career-high 18 home runs in just over 400 at bats, third among AL rookies, and his 49 RBIs were ninth among AL rookies.

In November 1999, he was traded by the Tigers with Al Webb, Frank Catalanotto, Francisco Cordero, Bill Haselman, and Justin Thompson to the Texas Rangers for Juan González, Danny Patterson, and Gregg Zaun.

Longtime Boston Globe columnist Bob Ryan—who said of Kapler “He’s as smart as any player I’ve ever met”—nicknamed him “The Body”. He is an avid weightlifter; his body fat count was reported in 2000 to be at 3.98%, and in 2012 to be at 3.5%. He was on the cover of several fitness magazines, and became renowned for being the focus of an entire K-Swiss shoe campaign before he had even reached the pros. His blog discusses fitness, nutrition, health and leadership.

Kapler hit two home runs on Opening Day in the 2000 season for the Rangers, becoming the first player to homer in his first two at bats as a Ranger. In July he homered in four straight games, tying a club record. He then had a team-record 28-game hitting streak later that season, which was also a major league high for the season. On July 30, he was named the American League’s Player of the Week. In 2000, he batted .302/.360/.473 with 32 doubles (second on the team), 14 home runs, and 66 RBIs in 444 at bats, hitting .344 in the second half of the season. On defense, playing primarily center field, he tied for second among AL outfielders with 4 double plays.

In 2001, he hit 17 home runs, scored 77 runs, had 72 RBIs, and stole 23 bases (leading the team) in 29 attempts. Kapler batted .267/.348/.437. He made just one error in 344 total chances for a .997 fielding percentage, second-best in the AL, and his 8 assists tied for fourth-most of any AL center fielder.

In July 2002, the Rangers traded Kapler, with Jason Romano and cash, to the Colorado Rockies for Dennys Reyes and Todd Hollandsworth. Playing for the Rockies in 2002, he batted .311/.359/.445 in 119 at bats. In 2002 between Texas and Colorado, he batted .279, and .357 with runners in scoring position, as on defense he had 10 outfield assists.

On June 28, 2003, the Boston Red Sox acquired Kapler from the Rockies. He batted .292/.349/.449 for the Red Sox, in 158 at bats.

In 2004, when Shawn Green of the Dodgers announced that he would not play on Yom Kippur, the Boston media asked Kapler if he would do the same thing. Kapler called a Boston-area rabbi for advice. With the Curse of the Bambino still hanging over Red Sox fans’ heads, the rabbi reportedly said: “Do it! We need all the help we can get!” Kapler decided to play.

Kapler played a career-high 136 games in 2004, hitting 6 home runs and driving in 33 runs in 290 at bats, as he primarily played right field. He batted .272. He also led the team with 6 outfield assists.

Less than one month after the Red Sox dramatic 2004 World Series victory, Kapler departed to play for Japan’s Yomiuri Giants. He received a $2 million deal plus a $700,000 signing bonus, compared to the $750,000 salary he had received from the Red Sox. Driven by the memory of an elementary-school report that he had written about Japan, he felt it was time for a change. “I tend to make emotional decisions,” he said. “I did it more for the life experience than anything else. And ever since I wrote that report, I’ve been fascinated by everything that an 8-year-old associates with a country far, far away.” He struggled in 38 games in Japan, and was placed on the inactive list by Yomiuri in the 2005 mid-season.

Kapler was re-signed by the Red Sox in July 2005, just a few hours after clearing Japanese Central League waivers. In September 2005, Kapler ruptured his left Achilles tendon while rounding the bases after a home run by teammate Tony Graffanino. He had surgery, which ended his season.

Kapler has been given the nickname Hebrew Hammer due to his frequent longball hits, along with his muscularity and the fact that he is Jewish. It later became the nickname of Ryan Braun, who is also Jewish, and was Kapler’s teammate on the Brewers. On August 8, 2005, while playing for the Red Sox, Kapler took the field in the 9th inning along with Kevin Youkilis and Adam Stern, setting a “record” for the most Jewish players on the field at one time in American League history and the most in Major League Baseball history since four Jews took the field for the New York Giants in a game in 1941.

In June 2006, Kapler came back from his injury. That season, he had his best on-base percentage in 5 years (.340), hit .316 with 2 out and runners in scoring position, and played error-less outfield for the second year in a row.

Kapler announced his retirement from professional baseball on December 12, 2006.

In 2006 he was inducted into the Southern California Jewish Sports Hall of Fame.

He served the Boston Red Sox as manager of their Single-A affiliate, the Greenville Drive, for one season in 2007. The team went 58–81, and finished in 7th place in the South Atlantic League Southern Division.

On September 20, 2007, after one season as a manager, Kapler announced that he would like to return to play Major League Baseball in 2008. On the decision, Kapler said “I miss the battle. I still need to be on the field as a player.” On December 20, Kapler signed a one-year, non-guaranteed contract with the Milwaukee Brewers that paid $800,000 when Kapler made the roster.

While Cameron served a 25-game suspension to start the season for twice testing positive for a banned stimulant in the fall of 2007, Kapler made the club, and began to see action in center field. On April 5, 2008, he hit the first pinch-hit home run of his career for Milwaukee in the 7th inning of a game against the San Francisco Giants. Kapler started the season as the Brewers’ hottest hitter, going 11-for-26 with 4 home runs and 11 RBIs.

In 2008, with his career 69th home run he passed Art Shamsky and Lou Boudreau for 9th on the all-time list for home runs by Jewish major leaguers. Kapler was the unanimous winner of the 2008 Jewish Comeback Player of the Year award. Through 2018, he was 5th all-time in career stolen bases (behind Brad Ausmus) and 9th all-time in career doubles (behind Sid Gordon) among Jewish major league baseball players.

On January 12, 2009, Kapler signed a one-year contract with the Tampa Bay Rays for $1,000,018. The extra $18 was chosen because it represented the symbol for “life” in Judaism.

He started the season platooning in center field with Matt Joyce, in place of Rays center fielder B. J. Upton, who had offseason surgery on his left shoulder and was not ready for Opening Day. On April 13, Kapler struck out against New York Yankee outfielder Nick Swisher. Kapler then began to platoon in right field with Gabe Gross. In June, he tied a club record shared by Jose Canseco and Julio Lugo, with home runs in four straight games. Kapler ended the season at .354 with 4 homers in 82 at bats against left-handers. In this role he almost broke up Mark Buehrle’s perfect game on July 23, 2009. Leading off the ninth inning against the White Sox, he was robbed of a home run by a leaping DeWayne Wise, a ninth inning defensive replacement.

Through July 10, despite a slow start, Kapler had the best slugging percentage of his career (.505), and was batting .320 with a 4 home runs in 75 at bats and a .680 slugging percentage against left-handers. As of July 10, 64% of his hits in 2009 had been for extra bases, which would be first in the major leagues for a player with at least 100 plate appearances (Kapler had 129).

Kapler was re-signed by the Rays on October 27, 2009, to another one-year contract, this time for $1.05 million. Over 2008–09, Kapler hit .304 against left-handers with a .577 slugging percentage, 9th-best in the Major Leagues. “Over the past two years, Kap has been one of the best in baseball against left-handed pitching”, said Rays executive vice president of baseball operations Andrew Friedman. “Because he’s also a plus defensive outfielder, he’s become a tremendous asset here. His value even extends beyond the field; his knowledge and presence make him a positive influence on our younger players.”

Heading into spring training in 2010, Kapler was likely to platoon in right field with Joyce. However, he appeared in only 59 games that season, hitting a career-worst .210 with only two home runs as he battled right hip flexor and right ankle injuries, and became a free agent after the season.

On January 18, 2011, Kapler signed a minor league contract with the Los Angeles Dodgers. He also received an invitation to spring training. Kapler was released on March 31, in the team’s last cut of spring training.

From 2012 to 2013, Kapler worked closely with technology startup Egraphs, which focused on electronic personalized autographs, and which was licensed with MLB and the NBA. In spring 2013, Egraphs closed operations.

Kapler coached for the Israeli national baseball team in the 2013 World Baseball Classic qualifier in September 2012. Israel lost to Spain in extra innings in the Pool Finals, and did not qualify to play in the World Baseball Classic.

In the summer of 2013, Kapler became an analyst for Fox Sports 1 cable network. He appeared frequently on the network’s Fox Sports Live program from the network’s debut on August 17, 2013, as well as MLB Whiparound from the program’s inception in March 2014. Two of his segments were “Saberclips”, in which he explained advanced statistics and sabermetrics used in baseball, and also “In the Cage”, in which he shared advice with young baseball players as to how to train when they hit the batting cage. He also worked as a writer and analyst at Baseball Prospectus, which is devoted to the sabermetric baseball analysis.

On November 7, 2014, Kapler became the Dodgers’ director of player development. In that position, Kapler had all the affiliates in the Dodgers farm system, as well as the major league team, switch to serving entirely organic food, and take junk food out of the clubhouse. Kapler, known as a proponent of advanced statistics and healthy food, explained his flexible general approach, saying:

In 2015, a 17-year-old accused a Dodger minor league player initially of videotaping a fight in which two women whom she had met through Facebook hit her while with her in a hotel room of a player. The accuser emailed her complaint to Kapler. He sought direction from Dodgers’ lawyers and human resources personnel and then replied quickly with a phone call, apologized, and offered to help in any way she needed. A week later—when the accuser was arrested for shoplifting—she added the second accusation to police, though she did not communicate it to Dodgers personnel, that at the time a Dodger minor league player engaged in alleged sexual assault by briefly putting his hand under her bra and down her pants; the accuser then declined to cooperate with the police, and no charges were filed. Kapler reported the accusation of the videotaping of the assault to Dodgers personnel, who did not report it to Major League Baseball. Kapler has apologized for his handling of the allegations.

On October 30, 2017, the Philadelphia Phillies announced that they had hired the 42-year-old Kapler as their new manager, the 54th in team history. Phillies General Manager Matt Klentak and principal owner John Middleton said that what most impressed them during Kapler’s interview, which included people across several team departments, were his level of preparation and his people skills, evidenced by his ability to connect with each one of the groups.

In 2018, Kapler became the eighth Jewish manager in MLB history. He joined Bob Melvin, Brad Ausmus, Jeff Newman, Norm Sherry, Lou Boudreau, Andy Cohen, and Lipman Pike.

In 2019 he managed the team to an 81–81 (.500) record, Philadelphia’s best record since 2012 when they also had a .500 record. Following the season, Kapler was fired. Joe Girardi succeeded him, and managed the team to a 28–32 (.467) record the following season.

In November 2019, the San Francisco Giants hired Kapler as their 37th manager, succeeding the retiring Bruce Bochy. He inherited a team that had a win-loss record of 77–85 (.475) in 2019.

In 2020, Kapler hired three hitting coaches who were young enough to still be players, the Giants veteran hitters raved about how well prepared they were going in to games with the new approach, Kapler platooned whenever he thought he could get an advantage, and the resulting turnaround with the Giants hitters was significant. While the team had ranked 28th, 29th, and 29th in runs the prior three seasons, under Kapler they finished 8th in runs, averaging 4.98 runs per game, up from 4.19 the prior year and 3.72 in 2018. Kapler used 1.25 pinch hitters per game, more than any other manager in major league baseball. Both new and veteran players responded positively to Kapler, and a number of them highlighted his steadiness as a factor in turning around slumping years. In the pandemic-shortened 2020 season, he managed the team to an 29–31 (.483) record that far exceeded expectations in what was expected to be a rebuild year, and surpassed the record of Kapler’s former team, the Phillies. The Giants felt just short of making the playoffs, which they would have made had they won their last game of the season, which they lost by one run. They ended up with the same win-loss record as the Milwaukee Brewers, but it was the Brewers that advanced to the playoffs as they had a better in-division record.