Juan Then signed with the Seattle Mariners as an international free agent on July 2, 2016, when he was 16-years old for $77,000. The New York Yankees acquired Then along with southpaw JP Sears from the Mariners in exchange for reliever Nick Rumbelow on November 18, 2017.
The San Francisco de Macoris, Dominican Republic native fired 61.1 innings over a span of 13 starts. Then made a strong impression in his first season in professional baseball by collecting 56 strikeouts while just permitting 12 walks. He pitched to a 2-2 record with a 2.64 ERA and .220 opponent batting average against.