Senator Mabanag was born on July 4, 1886 in San Fernando, La Union, to Liberato Mabanag of Calamanugan, Cagayan and Manuela Razulo of Bangar, La Union.
For being studious, bright and industrious, Mabanag supported himself while getting his higher education. He was clerk in the defunct Executive Bureau from 1903 to 1912 and finished his Bachelor of Arts at the University of Sto. Tomas in 1908. He was senior clerk when he obtained his Bachelor of Laws degree from La Jurisprudencia in 1912 and passed the bar examinations the same year.
He opened his law firm in Lingayen, Pangasinan and attracted big clients in the province. His popularity and his ability became his qualifications for a three year service as Lingayen municipal councilor from 1916 to 1919. He ran for senator and won overwhelmingly to represent the second senatorial district of la Union in 1922 and re-elected in 1931.
Upon the establishment of the Commonwealth government on November 15, 1935, President Manuel L. Quezon appointed him Judge of the Court of First Instance of Iloilo up to 1938 and later named as city fiscal of Manila until 1945.
He ran anew for the Senatorial election and won in 1946 but he lost in the 1949 election. As a distinguished Senator he became the chairman of the Senate Blue Ribbon Committee from August 1952 to August 1953 and in 1953 elections he ran and won again as Senator under the Magsaysay ticket. He was also member of the following committees; Corporations and Franchises, Codes and Constitutional amendments, Government Reorganization, Investigation, Justice, Labor and Immigration, National Defense and Security, National Enterprises, provincial and Municipal Governments and Cities, Transportation and Public Service.