Mario K. Castillo serves as Chancellor of Lone Star College System. He took up the title of General Counsel at the System in 2015. From 2016 to 2017, he served as Vice Chancellor and General Counsel. From 2017 to 2023, he served as the Chief Operating Officer and General Counsel. From 2020 to 2021, he served as interim President at Lone Star College - Kingwood in addition to his regular duties. In 2023, he assumed the title of Chancellor of Lone Star College System.

As Chancellor, Mario is charged with determining the direction and long-term vision of the System. As the Chief Executive Officer, he oversees all System functions. Mario is deeply dedicated to the System's community and has reshaped processes and procedures to be student-centric and student-informed. Mario understands that you cannot be what you cannot see-he therefore ensures he meets students where they are. Mario provides numerous student scholarships and internship opportunities, and he regularly meets with students to offer career advice.

By putting them first, Mario has guided the College toward improving retention and outcomes for students. In 2025, he introduced Do the Thing & Be the Bridge: 2025-2030 as the College's strategic plan. This strategic plan establishes four primary goals: increasing financial resilience, promoting academic excellence, developing the College's culture and talent, and building bridges for students to prosper. These goals are supported by twenty-four key performance indicators, each assigned to a Chancellor's Cabinet member. The goals and KPIs guide the College toward achieving its mission: to provide transformational educational opportunities that help individuals get from where they are to where they want to be.

Prior to joining the System, Mario served as a Partner at Monty & Ramirez, LLP practicing Labor and Employment Law. During this time, he represented the System as outside counsel in various matters. His legal experience includes litigation on cases in state and federal courts, various appellate courts, and the Texas Supreme Court; serving as a briefing attorney for the Honorable Felix Recio of the Southern District of Texas. Mario's peer-reviewed articles have been published by the Baylor School of Law and the University of Houston Law Center. He also maintains Board Certification in Labor and Employment Law by the Texas Board of Legal Specialization.

Mario received his Juris Doctorate from the Maurer School of Law at Indiana University in Bloomington and received his Bachelor of Arts in Government from the University of Texas in Austin. Mario is a first-generation high school (on his mother's side), college, and law school student as well as a first-generation American. He enjoys overly ambitious home improvement projects, marathon running, and is an avid reader.