The victory of Admiral Dewey over the Spaniards in Manila on August 13, 1898 brought about general changes in Mindoro. A general primary school system with English as the language of instruction was established. Calapan port was opened to inter-island commerce. The U.S. Army Signal Corps connecting Calapan and Batangas installed a series of military cables.
Land telegraph for public use was also installed in Calapan and Naujan. With the construction of provincial road along the east coast, the most important towns of the province were connected with one another. Free trade was established between the U.S.A. and the Philippines that brought about significant changes in the economy of Mindoro. Infrastructure and economic measures were adopted which induced massive wave of migration to the island.
Changes in the affairs of the local government also took effect in the island. Mindoro was made a sub-province of Marinduque on June 23, 1902 by virtue of Act No. 423 of the Philippine Commission. On November 10, 1902, Act No. 500 separated Mindoro from its mother province, thereby organizing its provincial government. The same Act provided further that "the province shall consist the main island and the smaller islands adjacent thereof, including the islands of Lubang, Caluya and Semirara. Puerto Galera was made the seat of government, with Captain R.C.
Offley as the first civil governor. In 1907, the province was allowed to elect its first delegate in the person of Don Mariano Adriatico. Mindoro was finally declared a regular province in 1921.
In the years following the invasion of Mindoro by the United States Forces, there had been a considerable increase in population due to the pouring into the highly underpopulated island of a massive influx of new settlers. For the first time, the development and cultivation of the island's interior was made possible.
The structure of society and the distribution of landholdings were likewise altered. The minority policy of the Americans was adopted, uplifting the Mangyans to the Filipino majority's level of civilization through special educational regulation and separate settlements.
