Keynan Anthony Middleton grew up in Portland, Oregon, attending Milwaukie High School and Lane Community College. Prior to joining the Mariners, he appeared in four major league seasons with the Los Angeles-AL from 2017 to 2020, going 6–1 with 9 saves and a 3.48 ERA (37 ER, 95.2 IP), 96 strikeouts and 40 walks. He was drafted in the 3rd round of the 2013 MLB First-Year Player Draft out of Lane Community College at 19-years-old. Since signing with the Seattle Mariners for the 2021 season, he has played in 13 games with 3 saves and a 3.