He played in Major League Baseball (MLB) from 2013 to 2024 for the Tampa Bay Rays, Toronto Blue Jays, and Los Angeles Dodgers.

[2] The Tampa Bay Rays selected Kiermaier in the 31st round of the 2010 Major League Baseball (MLB) draft.

[6] Rays general manager Andrew Friedman commented that the club included Kiermaier on the major league roster late that season specifically for his defense in center field.

He played two innings in the 2013 American League Wild Card Game against the Cleveland Indians, which was his only postseason appearance that year.

[9] He finished 2014 hitting .263 with 10 home runs in 108 games[10] and was one of the finalists for the Gold Glove Award for American League right fielders.

[11] In a game at Tropicana Field against the Kansas City Royals on August 29, 2015, Kiermaier was involved in a failed but humorous attempt to rob designated hitter Kendrys Morales of a home run.

[12] Morales hit a fly ball which appeared to be clearing the center field fence for a home run.

Kiermaier leapt and momentarily perched himself upright on the top of the fence and waited to catch the ball before it landed.

The ball had deflected off one of the catwalks suspended from the roof, and, in accordance with park rules, the umpires awarded a home run to Morales.

In a 5–4 loss to the Detroit Tigers on May 21, he fractured two bones in his left hand while attempting to catch a sinking line drive off the bat of James McCann.

[22] Despite missing significant time, Kiermaier was once again one of the best defenders in all of baseball, leading all center fielders with 25 Defensive Runs Saved (DRS) despite playing in almost 400 innings less than the next player on the list, Kevin Pillar.

[38] In game 3 of the American League Division Series against the Houston Astros, Kiermaier hit a three-run home run.

[39] By the 2020 season, Kiermaier had become the longest tenured Ray and the only active player who had been managed by franchise legend Joe Maddon.

[42] According to FanGraphs, Kiermaier led all center fielders in UZR (7.7), UZR/150 (30.7), and assists (6), while being second in defensive runs saved (10) behind only Byron Buxton (11).

[45] In the American League Championship Series against the Houston Astros, Kiermaier got hit in the hand by a pitch and was unable to start in games 4, 5, and 6.

[48] On April 23, 2022, a day after his 32nd birthday, Kiermaier hit his first career walk-off home run, in the 10th inning of a game against the Red Sox, to win 3–2.

[30] On November 5, 2023, Kiermaier won his 4th career Gold Glove Award, edging out the other two American League center field nominees, Julio Rodriguez of the Seattle Mariners and Luis Robert Jr. of the Chicago White Sox.

[57] On July 30, 2024, the Blue Jays traded Kiermaier to the Los Angeles Dodgers in exchange for Ryan Yarbrough.

[61] On February 8, 2025, Kiermaier was hired to join the Toronto Blue Jays' front office as a special assistant.