Isiah Gilliam was drafted by the New York Yankees in the 20th round of the 2015 draft out of Chipola College in Marianna, FL. The switch-hitting outfielder was signed for an above-slot bonus of $550,000.
To start off his career, the Yankees sent Gilliam to the Yankees 2 team in the Rookie-level Gulf Coast League. He played in 42 games and put up a .296/.359/.415 slash line. He also hit one home run with 11 doubles and one triple. He ended up driving in 23 runs. Gilliam split time between left field (11 games) and right field (17 games).
Gilliam was moved up to the Rookie-level Pulaski Yankees where he played in 57 games. He produced a .239/.301/.440 slash line with 10 home runs, 14 doubles, and 33 RBI. With the P-Yanks, Gilliam played mostly right field (45 games) while playing sparingly in left field (5 games).
Gilliam was promoted to the Charleston RiverDogs of the low Class A South Atlantic League. Gilliam would spend the entire season for the RiverDogs and was the most consistent hitter in the lineup.
In 125 games, he posted a .275/.356/.468 slash line with 15 home runs, 33 doubles, and four triples. He also knocked in 85 runs. Gilliam spent most of his time in right field with 74 games played while playing 10 games in left.
Gilliam was named South Atlantic League player of the week three times.