Abad was a Dean's Lister at the Ateneo de Manila University, where he earned his law degree. He is a member of the Fraternal Order of Utopia. Utopia currently has two members who are seated in the highest tribunal of the Philippines, one is Justice Abad, the other is Justice Arturo Brion. He first engaged in private practice at the Jose W. Diokno Law Office in 1968, and then he joined the government, working as Technical Assistant (1969–1973) and Associate Attorney (1974–1975) at the Supreme Court, supervised by then Chief Justice Fred Ruiz Castro.
In 1975, he joined the Office of the Solicitor General (OSG). In 1985, he was promoted to Assistant Solicitor General, a post he held for about a year before setting up his own law firm.
He has rendered free legal aid for the Free Legal Assistance Group, Department of Social Welfare and Development, and the Angels of Hope Orphanage, Pulong Bunga, Silang, Cavite. He has conducted weekend training for lay and religious catechists for the Archdiocese of Manila.
From 1988-1990, he worked as legal consultant for the presidential committee on the nuclear power plant under the late Justice Secretary Sedfrey Ordoñez. Later, he worked as counsel for the Equitable Banking Corporation and its officers and branch managers during the impeachment trial of former President Joseph E. Estrada.
Prior to his retirement, he penned the High Court's ruling on the contentious Cybercrime Prevention Act of 2012, a landmark decision.
The late Chief Justice Roberto C. Concepcion, then University of Santo Tomas (UST) faculty of civil law dean, recruited Abad from the OSG in 1978 to teach political law at the UST. Abad also taught constitutional law, administrative law, election law, law on public corporations, and public international law. He became a bar reviewer in political law.
He also served as Dean at the University of Santo Tomas Faculty of Civil Law.
He authored two books: Practical Book in Legal Writing (2002) and Fundamentals of Legal Writing (2004). He was a contributing staff editor in the Supreme Court Reports Annotated (SCRA) from 1972 to 1996.
He conducted a seminar and workshop in legal writing and research in 2007 for the attorneys and investigators of the Office of the Ombudsman upon the invitation of the Philippine Judicial Academy, the United States Agency for International Development, the Rule of Law Effectiveness, and CD Technologies Asia, Inc. He lectured to the research attorneys of the Sandiganbayan and the Court of Tax Appeals regarding the preparation of judicial memoranda.
He was widowed by his first wife Liliabeth Abad, with whom he has four children. He is now married to Victoria Martinez, a lawyer.Disini v. The Secretary of Justice" (18 February 2014)] — upholding most provisions of the Cybercrime Prevention Act of 2012 as constitutional