Seymour was born in the Bahamas and drafted in the 7th round of the 2014 draft by the Miami Marlins out of American Heritage High School in Florida. He was put into the Gulf Coast League, and he flashed his incredible speed, hitting .245/.333/.265 with just one extra base hit, but 11 stolen bases and 12/27 BB/K over 112 plate appearances. He played 19 of his 26 games in center field and 7 at shortstop.
Seymour was bumped up to Batavia in the short-season New York-Penn League in 2015. He hit for a better average, but his overall statistical line was not tremendously different.