The Kansas City Royals selected Merrifield in the ninth round of the 2010 Major League Baseball draft.[7] He signed with the Royals and made his professional debut that season with the Burlington Bees.[8][9] In 47 games he hit .253/.317/.409 with five home runs and 26 RBIs. In 2011, Merrifield played for the Wilmington Blue Rocks where he batted .262 with five home runs and 36 RBIS. In 2012, with both Wilmington and the Northwest Arkansas Naturals, he compiled a .258 batting average with nine home runs and 44 RBIs in 125 games between both teams. He spent 2013 with Northwest Arkansas where he batted .270/.319/.391 with three home runs and 43 RBIs in 94 games.[10]
Merrifield returned to Northwest Arkansas to start 2014 and was promoted to the Omaha Storm Chasers during the season.[11] In 120 games between the two clubs, he slashed .319/.371/.470 with eight home runs and 49 RBIs. In 2015, Merrifield played for Omaha where he posted a .265 batting average with five home runs and 38 RBIs in 135 games.[10] He returned to Omaha to start the 2016 season.
Merrifield made his major league debut with the Kansas City Royals on May 18, 2016, instantly batting in the top third of the lineup and soon taking the starting second baseman job from Omar Infante.[12] His first Major League hit came off of David Price.[13] On June 13, Merrifield hit both his first major league triple and home run against the Cleveland Indians. In early July 2016, a song and video tribute to Merrifield titled "Cool Whit" received coverage on local Kansas City radio and TV news.[14] He was optioned back to Omaha in July and recalled in September. In 69 games for Omaha he batted .266 with eight home runs and 29 RBIs, and in 81 games for Kansas City he compiled a .283 batting average with two home runs, 29 RBIs, and 22 doubles.[15]
Merrifield began the 2017 season with Omaha, but was recalled in April after nine games and spent the remainder of the season with Kansas City. With the Royals, he hit .288 in 145 games with 19 home runs and 78 RBIs. He also led the American League with 34 stolen bases, the fewest total for a league leader since Luis Aparicio led the AL with 31 in 1962.[16] In 2018, Merrifield hit .304/.367/.438 and led the majors in hits (192) and stolen bases (45).[17]
In 2019, Merrifield batted .302/.348/.463 and led the major leagues in games (162), at bats (681), singles (139), triples (10), and line drive percentage (28.2%), while stealing 20 bases and leading the majors in caught stealing (10).[18][19]
Overall with the 2020 Kansas City Royals, Merrifield batted .282 with nine home runs and 30 RBIs in 60 games.[20]
In 2021, Merrifield hit .277/.317/.395 with 10 home runs and 74 RBIs. He tied for the MLB lead with 42 doubles, and led the American League with 40 steals. He also tied for the major league lead in sacrifice flies, with 12.[21] On defense, he led all major league second basemen with 283 putouts, 103 double plays and a 4.77 range factor. He won a Fielding Bible Award for his defensive excellence.[22] Merrifield played in a franchise-record 553 consecutive games for the Royals between June 2018 and July 2022.[23]
On August 2, 2022, Merrifield was traded to the Toronto Blue Jays for Max Castillo and Samad Taylor.[24] He was among ten Royals players who were in violation of Canada's COVID-19 vaccination requirement and unable to travel to Toronto for a four-game weekend series right before the All-Star break two weeks earlier in July.[25] On August 4, 2022, Merrifield announced that he was now vaccinated and would be able to play in the team's first game in Toronto since being acquired.[26]
In 2022 he batted .250/.298/.375 in 550 plate appearances, and was the only qualified batter in the major leagues who was not hit by a pitch all year.[27] He became a free agent following the 2023 season.
