[1] Born in Chicago, Illinois to a Mexican-American father and a Puerto Rican mother,[2] Castillo was the first member of his family to finish college.

[3] Castillo earned a Bachelor of Arts degree in political science in 1976 from Loyola University in Chicago, working nights as a clerk at the Illinois Circuit Court of Cook County to put himself through school.

[5] In 1984, he was named an Assistant United States Attorney for the Northern District of Illinois.

He worked as an Assistant United States Attorney until 1988, when he became a regional counsel for the Mexican American Legal Defense Fund.

[5][4] Following his retirement from the judiciary, Castillo rejoined private practice in the Chicago office of Akerman LLP.