He played college football for the Oklahoma Sooners, winning an NCAA national championship in 1985, when he also won the Lombardi Award and was the UPI Lineman of the Year.

In 2004, Casillas became the second Hispanic (his father is Mexican and his mother comes from Irish and Native American descent) to be inducted into the College Football Hall of Fame.

[8] He returned to the team with a new attitude and in time to start the regular season, where he made 111 tackles and was named a Pro Bowl alternate.

He exploded in 1989, becoming one of the best players at his position in the league with 152 tackles (still a team record for a defensive lineman) and was named second-team All-Pro and a Pro Bowl alternate.

In 1990, he held out in a contract dispute from head coach Jerry Glanville's first Falcons training camp,[9] causing him to lose his starting job to rookie Tory Epps.

The problems escalated from there on, he eventually missed a 44–24 loss to the Los Angeles Rams after failing to catch the team flight.

[12] During his five-year span in Atlanta, Casillas had 478 tackles, the most ever by any Falcons defensive lineman and fourth best overall in franchise history.

[15] As part of Casillas's release from the Chiefs, he agreed that he would not sign in 1994 with any of the other AFC West divisional rivals or with the Dallas Cowboys.

In 1994, he signed with the New York Jets after a bizarre spring and summer in which he practiced only sporadically in minicamp with the Chiefs and was eventually released by the team.