[3][4] After graduating from law school, she worked with the Nursing Home Task Force of the Texas Attorney General's office and as an English teacher in San Antonio.

She joined the Mexican American Legal Defense and Educational Fund (MALDEF) in 1979, serving as a trial and appellate lawyer in federal and state courts in class action impact civil rights cases.

While at MALDEF she litigated significant cases affecting educational funding, disability rights, student disciplinary policies, access to special services for English-language learners, and racially hostile environments.

[5][6] She served the nation for eight years in this capacity, where she oversaw a staff of approximately 850 in implementing governmental policy for civil rights in American education.

Within the first two years, her office increased the number of illegal discrimination complaints resolved by 20%; more than a third of the cases were disposed of without adversarial proceedings based on voluntary corrective action.

In 2002, Cantú co-founded the Mexican-American Legislative Leadership Foundation, a not-for-profit organization to encourage students to gain experience on staff to the Texas Legislature.