After graduating from Harvard Law School in 2007, Mateo worked for five years in Washington, D.C. at the nationally recognized law firm of Covington and Burling. During that time, Mateo practiced tax law and conducted internal investigations of large publicly-traded corporations, while at the same time taking on significant pro bono cases ranging from representing indigent defendants in landlord-tenant court to filing a clemency petition on behalf of a mentally disabled person on death row in Virginia. In 2008 at the height of the financial crisis (between his time at Covington), Mateo also clerked for Judge Elizabeth S. Stong at the U.S. Bankruptcy Court in the Eastern District in New York in Brooklyn.

Starting in 2013, Mateo worked for over three years at a premier Hawaiʻi law firm doing litigation, internal investigations, real estate, and multi-million transactional work. In September 2016, Mateo became Legal Director at the American Civil Liberties Union of Hawaiʻi, where he managed their impact litigation docket on a wide range of civil rights and liberties issues, including free speech, disability rights, privacy and technology, due process, and equal protection. At the ACLU, Mateo defended and advanced the civil rights and liberties of everyone in Hawaiʻi, but particularly of those most marginalized by the legal system and society. Mateo is also a lecturer on civil rights and liberties at the University of Hawaiʻi at Mānoa at the William S. Richardson School of Law. After four and a half years, Mateo left the ACLU to launch Caballero Law to marry his expertise in civil rights and liberties with his passion for justice and for helping others through complex legal problems.