Prior to the 2012 season, Simmons was the Braves' fourth-best prospect according to Baseball America.[5] He was ranked 92nd in all of baseball.[6] During spring training he competed with Tyler Pastornicky for the Braves starting shortstop job.[7][8] On May 30, 2012, the Braves called Simmons up,[9] and he made his Major League debut on June 2 against the Washington Nationals.[10] He got his first Major League hit, a double, on June 3.[11] Simmons was named National League Rookie of the Month for June 2012 after hitting .333 with six doubles, three home runs, and 14 RBIs in June. He led all NL rookies in batting average and on-base percentage.[12] He also won praise for his outstanding defense.[13][14] On July 8, 2012, during a game against the Philadelphia Phillies, Simmons suffered a non-displaced fracture of the fifth metacarpal on his right hand while sliding head-first into second base.[15] On July 9, he was placed on the 15-day disabled list.[16]
In the 2012 National League Wild Card Game against the St. Louis Cardinals, Simmons hit an eighth-inning fly ball to left field that dropped in between Cardinals shortstop Pete Kozma and left fielder Matt Holliday. Simmons was believed by many to have been entitled to a single, but left field umpire Sam Holbrook cited the infield fly rule[17][a] and called Simmons out. Angered Braves fans littered the field with trash, causing a 19-minute delay and a threatened forfeit.[21][22][23] The Braves lost the game 6–3 and were eliminated.
Simmons had his first full season in 2013, playing in 157 games. He finished the 2013 season with the highest Defensive Runs Saved ever recorded in a year[24] and received the Gold Glove,[25] the Fielding Bible Award,[26] and the Rawlings Platinum Glove Award[27] for his defensive work that season.
On February 20, 2014, the Atlanta Braves signed Simmons to a seven-year deal worth $58 million.[28][29] In 2014 he batted .244/.286/.331. At the end of the year, Simmons won the Gold Glove[30] and Fielding Bible[31] awards for his position.
In 2015 he batted .265/.321/.338. He had the lowest number of pitches per plate appearance in the major leagues (3.27).[32]
In 2017, Simmons hit .278/.331/.421 with 14 home runs and 69 RBIs, and had the lowest strikeout percentage of all major league baseball players (10.4%).[38] Simmons finished 8th in the American League Most Valuable Player voting. He received his third Gold Glove Award as well in 2017.[39] His Defensive Wins Above Replacement in 2017 was 5.0, third-highest of all time and highest since 1917.[40]
In 2018, he batted .292/.337/.417 with 11 home runs and a career-high 75 RBIs, a major-league-leading 28 infield hits, had the lowest strikeout percentage of all major league baseball players (7.3%) for the second consecutive year, had the lowest number of pitches per plate appearance in the major leagues (3.30), and led the majors in pull percentage (51.0%).[41][42] He also won a Gold Glove for the second consecutive season.[43] As of 2018, he led the major leagues in career at bats per strikeout (10.32).[44] He was placed on the disabled list on May 21, 2019, with an ankle injury.
On September 22, 2020, Simmons announced he was opting out of the remainder of the 2020 season. At the time of his opt-out, Simmons had slashed .297/.346/.356 with no home runs and 10 RBI.[45] While he initially attributed the opt-out to concerns relating to the COVID-19 pandemic, Simmons later revealed in February 2021 that he opted-out due to depression and suicidal ideations.[46][47]
Minnesota Twins
On January 31, 2021, Simmons signed a one-year, $10.5 million contract with the Minnesota Twins.[48] Simmons finished the 2021 season with a .223 batting average, 3 home runs, 31 RBI, 1 stolen base, and a 57 OPS+, the lowest of his career.
Chicago Cubs
On March 15, 2022, Simmons signed a one-year, $4 million contract with the Chicago Cubs.[49] In 34 games for Chicago, he batted .173/.244/.187 with no home runs, 7 RBI, and 4 stolen bases. Simmons was designated for assignment on August 6,[50][51] and elected free agency the following day.
Mumbai Cobras
On October 23, 2023, Simmons was drafted in the first round by the Mumbai Cobras, with the 8th overall selection, of the Baseball United inaugural draft.[52]
On December 26, 2023, Simmons announced his retirement from professional baseball.[53][54]
Despite his retirement, on January 13, 2024, Simmons was announced as part of the Curaçao Suns roster that would represent Curaçao in the 2024 Caribbean Series in Miami.[55]
^The infield fly rule states, in part, that the batter is out when, with first and second base occupied with fewer than two out, the batter hits a fair fly ball which is not a line drive nor bunt, and this fly ball can be caught by an infielder with ordinary effort. This was controversial in part because the ball landed 225 feet (69 m) from home plate, in the outfield grass, and was by far the deepest uncaught pop-up called infield fly since 2009, and as a result was so far from the bases that intentionally letting it drop could not have led to an unfair double play (the intention of the rule is to prevent such a play) as evidenced by the fact that both baserunners advanced easily.[18][19][20]