Private sector Mandanas started out by working for different companies. He was a supervisor of the Carlos J. Valdes & Co. from 1963 to 1965. He then served as an executive assistant to the president of the Far East Bank and Trust Company. In 1972, he was the president of the Fereit Realty Development Corporation until 1975 and as the Philippine Commercial Delegate of the Banque Francaise Du Commerce Exterior from 1975 to 1978. Then he became the managing director of the Manila & Hong Kong Capital Corporation from 1980 until 1987. He also served as the director for: Alpa Asia Hotels & Resorts Incorporated; United States Capital Corporation; Philippines-China Development Corporation; Oriental Pacific Equities (Hong Kong); Manila Taiwan Development Corporation; Apex Mining and Exploration Company and Rural Bank of Batangas. From 1987 to circa 1995, he also was the chairperson and president of the: Omnivest; Hedge Issues Management; Abacus Consolidated Holdings; Suricon Resources Corp.; and HIM Management Corp.
Government sector After the issuance of the Presidential Decree No. 1396, Mandanas served as the first general manager of the Human Settlements Development Corporation under then-president
Ferdinand Marcos. From 1995 to 2004, he served as
Governor of Batangas and concurrently president of the Federation of Regional Development Council of the Philippines. He also served as chairman of the Regional Development Council (Region IV-A, Southern Tagalog) from 1995 to 2004, the Regional Peace and Order Council (Region IV-A) from 1995 to 2001 and from 2003 to 2004, and
Calabarzon Coordinating Council from 2001 to 2004. From 2004 to 2013, he served as the representative for
2nd district of Batangas, holding positions such as former chairman of the Ways and Means Committee, vice-chairperson for Economic Affairs, and member for the majority in various committees including Banks and Financial Intermediaries, Bases Conversion, East ASEAN Growth Area, Energy, Foreign Affairs, Globalization and WTO, Government Enterprises and Privatization, Millennium Development Goals, National Defense and Security, and Trade and Industry. Additionally, he chaired Congressional Oversight Committees overseeing the Comprehensive Tax Reform Program, Physical Examination of Imported Articles, and Official Development Assistance, and was a member of the Safeguard Measures Act committee. During his tenure, he and
Bataan Governor
Enrique Garcia Jr. petitioned the
Supreme Court of the Philippines for a higher share of national internal revenue taxes for local government units, leading to the Mandanas-Garcia Ruling, named after them. In 2016, he was reelected to his second stint as Governor of Batangas. After being elected for a third and final time in 2022, Mandanas was elected vice governor of Batangas in the
2025 Philippine general election. As governor, Mandanas supported the expansion of
natural gas power plants in Batangas. ==Personal life==