According to his descendants, he retained his original surname then used the adoptive (Oirola) as his middle name, in honor of his foster parents.

In the Legislative, he was elected as a municipal councilor of Pandan from 1928 to 1934 and served as a Representative of Antique from 1934 to 1941; in the Executive branch, as Governor of Antique from 1951 to 1955 and as assistant and then acting Executive Secretary of President Diosdado Macapagal in 1964; and in the judiciary, as Associate Justice of the Supreme Court from 1964 to 1974.

[1][4][5] Zaldivar, along with Chief Justice Roberto Concepcion, were the dissenting voices during the deliberations on the issues arising out of the declaration of the Martial Law and the validity of the 1973 Constitution.

[6][7] Despite pressure to agree to a ruling that would legitimize the Martial law government of Ferdinand Marcos and his ideology of constitutional authoritarianism, Zaldivar and Concepcion persisted in their dissent.

[8] His positions as part of the Philippine Supreme Court emphasized civil and political liberties at a time when their value was being questioned, and he was known for being a proponent of the social sciences, religion, law, government, and statesmanship.