A self-employed businessman and owner of Casillas Insurance Agency, Marcelo S. Casillas is a life-long resident of San Antonio and of District 4 of the ACCD. In addition to his vast experience on the ACCD Board, Casillas served on the South San Antonio School Board from 1990-1998 and was re-elected four times. He was also served as president, vice-president, and secretary of the board.

Casillas formerly worked at Kelly AFB as a civil service aircraft mechanic. He is a fifth generation San Antonian with strong ties to the community. His personal and professional investment in San Antonio is displayed through his volunteer activities with the Knights of Columbus and the St. Bonaventure Catholic Church. Casillas graduated from South San Antonio High School in 1968.