Forsythe intended to take a medical redshirt in 2006, but entered a game after an early-season injury. As a Freshman, he appeared in 28 games and had a batting average of .189. He became the team’s starting third baseman as a Sophomore and improved to hit .347 in 61 games with nine home runs, 55 RBI and 18 stolen bases. He led the team in hits (78), doubles (16), steals and on-base percentage (.556). In post-season play, he hit .333 in the SEC Baseball Tournament and .267 in the Fayetteville Regional.

The San Diego Padres chose Forsythe in the supplemental first round (46th overall pick) of the 2008 MLB draft. He signed with the Padres on June 5, 2008 for a $835,000 bonus. He began his career that season in the low minors, appearing in nine games for the Arizona League Padres and three for the Eugene Emeralds.

Forsythe began 2009 with the Lake Elsinore Storm of the California League where he hit .322 in 66 games and was named to the mid-season All-Star Team. On June 25, 2009, he was promoted to the Double-A San Antonio Missions of the Texas League, and he hit .279 in 66 games for them.

In 2010, Forsythe was switched to second base and played the entire season for the Missions, hitting .253 in 107 games with 38 RBI and 17 steals. He also led the league in walks, despite missing over a month with a broken hand. He was invited to spring training by the Padres as a non-roster player in 2011 before he was assigned to the Triple-A Tucson Padres of the Pacific Coast League to begin the season.

Forsythe was called up to the MLB for the first time on May 4, 2011 and made his MLB debut that day, appearing as a pinch hitter in the fifth inning against Kevin Correia of the Pittsburgh Pirates and grounding out to shortstop.

On January 22, 2014, the Padres traded Forsythe, Brad Boxberger, Matt Lollis, Matt Andriese, and Maxx Tissenbaum to the Tampa Bay Rays in exchange for Alex Torres and Jesse Hahn.

In 2014, Forsythe slashed .223/.287/.329 with 6 home runs in 110 games with the Rays. He played First, Second, Third, Short, and Outfield in his first season with the Rays.

Forsythe married longtime girlfriend Ally Atkins on November 15, 2014. The two met while they were both attending the University of Arkansas. The couple’s first child, a son, was born in 2016.

In 2015, Forsythe had a career year. Slashing .281/.359/.444 while hitting a well-over career-high 17 home runs (previously 6) and 68 RBI (previously 26). He took advantage of an increase in at-bats and become one of the Rays’ offensive leaders. Forsythe’s numbers became even more impressive once turned into sabermetrics, the WAR measurements had Forsythe at 5.12, the second best 2nd baseman and 32nd best overall player.

On January 23, 2017, the Rays traded Forsythe to the Los Angeles Dodgers in exchange for José De León. On April 19, 2017, Forsythe was placed on the 10-day disabled list due to a fractured big right toe. The injury kept him sidelined for 30 games, and he did not rejoin the Dodgers until May 23. He struggled with his swing after his return, blaming his problems on his timing being thrown off by the injury. On the season, he played in 119 games, with a .224 batting average, six home runs and 36 RBI. Against right-handed pitching, he hit only .190, leading to him platooning with Chase Utley for much of the second half of the season. He also played a career high 42 games at third base, filing in for Justin Turner. He played in 14 of the Dodgers 15 playoff games, including all seven games of the 2017 World Series. He hit .297 with 11 hits in 37 at-bats and also walked nine times in the playoffs.

The Dodgers exercised his option for the 2018 season on November 6, 2017. Forsythe played 70 games with the Dodgers in 2018, batting .207 with 2 home runs and 13 RBI.

On July 31, 2018, Forsythe was traded to the Minnesota Twins, along with minor leaguers Devin Smeltzer and Luke Raley, for Brian Dozier. In the second half of the season with the Minnesota Twins, Forsythe appeared in 50 games batting .258 with no home runs and 14 RBIs.

On February 26, 2019, the Texas Rangers signed Forsythe to a minor league deal with a non-roster invitation to spring training. On March 22, 2019, the Texas Rangers announced that Forsythe had made the opening day roster as their utility infielder. Forsythe hit .227/.325/.353/.678 with 7 home runs and 39 RBI over 101 games for Texas in 2019.

On February 5, 2020, Forsythe signed a minor league deal with the Philadelphia Phillies. He was released on July 18, 2020.

On July 28th, 2020, Forsythe signed a deal with the Miami Marlins after being released by the Philadelphia Phillies. On August 3, Forsythe was selected to the active roster.