[12] On December 3, 2018, the Mets traded Kelenic, Jay Bruce, Gerson Bautista, Justin Dunn and Anthony Swarzak to the Seattle Mariners in exchange for Robinson Canó, Edwin Díaz, and $20 million.

Mather indicated that Kelenic and fellow Mariners prospect Logan Gilbert would start the 2021 season in the minor leagues, keeping them under team control for an extra year before free agency.

[23] After the comments became public, Kelenic said that the Mariners had offered him a contract extension and had told him multiple times that he would have appeared with the major league team in 2020 had he signed it.

[24][25] Despite impressing during spring training, with two home runs and a 1.256 OPS in nine games, Kelenic was assigned to the Tacoma Rainiers of the Triple-A West League to begin the 2021 season.

The next day, on May 14, Kelenic recorded his first MLB hit, a two-run home run in the bottom of the third inning for an estimated distance of 403 feet.

[31] Kelenic was called back up to the major leagues after the All-Star break and stayed on the roster for the remainder of the season, contributing to the Mariners' ultimately unsuccessful late-season playoff push.

[39] Kelenic was once again called up in late September, again hitting well[clarification needed] within the final month of the regular season as well as during the postseason, the Mariners' first since 2001.

[40][41] At the end of the 2022 season, Kelenic had a career batting average of .168, second to John Vukovich for lowest in history for batters with more than 550 plate appearances.

[45] Kelenic continued his spring training success into the major league season, hitting well enough to earn starts against left-handed pitchers and became the everyday left fielder.

On December 3, 2023, the Mariners traded Kelenic, Marco Gonzales, and Evan White to the Atlanta Braves for Jackson Kowar and Cole Phillips.

However, after Duvall struggled and injuries plagued the Braves' outfield, Kelenic became the starting everyday center fielder, and took the leadoff spot in the lineup.