Judge Fonseca was born and raised in South Texas. He graduated from the University of Notre Dame with a B.A. degree, with honors, in Political Science and Spanish Literature in 2003 and a J.D. degree, with honors, in 2009. Prior to attending law school, he served as a Paralegal Specialist for the Department of Justice Civil Rights Division in Washington, D.C. assisting in the prosecution of civil rights cases, with a particular focus on the prosecution of human trafficking offenses. He continued in public service while in law school through his work with the United States Attorney’s Office in McAllen, Texas after his first year and with Senator John Cornyn in the Judiciary Committee of the United States Senate after his second year. During his law school years, Judge Fonseca served in the Notre Dame Journal of Law, Ethics & Public Policy as note editor, and in the Notre Dame Legal Aid Clinic as a student-lawyer representing indigent clients in small estate, guardianship, and other probate matters. Judge Fonseca's career as an attorney began with his service as judicial law clerk for Judge Randy Crane of the U.S. District Court for the Southern District of Texas. He then continued his practice at Guerra & Sabo, PLLC (formerly Guerra, Leeds, Sabo & Hernandez, PLLC) where he specialized in civil defense, municipal law, and small business and non-profit representation. From 2019 to 2020, Judge Fonseca served as judge of the 464th District Court of Hidalgo County.

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On March 15, 2023, Governor Greg Abbott appointed Judge Fonseca to the 476th District Court of Hidalgo County, a court of general jurisdiction overseeing criminal, family and civil cases.