The news comes one day after a video surfaced of Mather sharing his thoughts on the service-time manipulation of prospects Jarred Kelenic and Julio Rodriguez, the expiring contract of Kyle Seager, and his displeasure with paying interpreters for foreign-born players.
"Like all of you, I was extremely disappointed when I learned of Kevin Mather's recent comments," Mariners chairman and managing partner John Stanton said in a statement.
"His comments were inappropriate and do not represent our organization's feelings about our players, staff, and fans.
"There is no excuse for what was said, and I won't try to make one. I offer my sincere apology on behalf of the club and my partners to our players and fans. We must be, and do, better."
Stanton will now act as president and CEO until a successor is chosen.
Major League Baseball also issued a statement following Mather's resignation:
"We condemn Kevin Mather's offensive and disrespectful comments about several players," the statement read, according to Ryan Divish of The Seattle Times. "We are proud of the international players who have made baseball better through their outstanding examples of courage and determination, and our global game is far better because of their contributions. His misguided remarks do not represent the values of our game and have no place in our sport."
On Feb. 5, Mather spoke with the Bellevue Breakfast Rotary Club, indicating Kelenic would not make it on the 2021 Opening Day roster and Rodriguez wouldn't reach the majors until 2022.
The video didn't leak until Sunday when it began to go viral, at which point Rodriguez tweeted out a meme:
Mather also said Seager was "overpaid" and 2021 would be the third baseman's last with the Mariners despite the former All-Star having a team option for 2022. Julie Seager, Kyle's wife, also sent out a tweet seemingly responding to Mather's comments:
Mather also stated he didn't like paying for the interpreter of Hisashi Iwakuma, who played six seasons with the Mariners and now serves as the team's special assignment coach.
The MLBPA issued a statement Monday morning condemning the Mariners and Mather:
"The (Mariners') video presentation is a highly disturbing yet critically important window into how players are genuinely viewed by management. Not just because of what was said, but also because it represents an unfiltered look into club thinking.
"It is offensive, and it is not surprising that fans and others around the game are offended as well. Players remain committed to confronting these issues at the bargaining table and elsewhere."
Kelenic seemed to take a victory lap of sorts on Monday, quote tweeting the MLBPA statement: