Santiago Castaño joined the Red Bulls Academy in September 2008 and played for New York's U-14, U-15, U-16, and U-18 teams.

Castaño made his professional debut at the age of 19 playing for New York Red Bulls II, the affiliate of his team, on March 28, 2015, in a 0–0 draw against Rochester Rhinos.

[1][2] Castaño was named to the USL Team of the Week for his performance for NYRBII on August 22, 2015, in a 2–0 victory over Toronto FC II, a match in which he made six saves and recorded his fourth clean sheet of the season.

[4] After leaving Red Bulls, Castaño left for Colombia, joining Socrates Valencia F.C in the Colombian Third Division.

On December 7, 2017, he scored from the penalty spot to help his club to a 2–1 victory over Fantasías Nueva York in the Copa Ciudad Pereira.