In Ponce, Mathias managed the banks of the Overman and Dede House, of which his father, Otto Georg Christian Degetau (Federico's grandfather), was a partner.

After the Spanish–American War, he was appointed by the military governor General Guy Vernor Henry as the Secretary of the Interior in the first cabinet formed under American rule in Puerto Rico, in 1899.

For this reason the political parties of the Island inscribed in their respective platforms the unanimous aspiration of the people to become and organized territory, with the certainty of soon being admitted as a State of the American Union.

[4]While serving Congress, Degetau was a member of the Committee on Insular Affairs, and submitted a bill to grant United States citizenship to Puerto Rico residents, which failed.

[5]In 1905, after traveling through Europe where he purchased a collection of paintings, Degetau moved to Puerto Rico and established his residence in the town of Aibonito where he managed a coffee plantation.

Isabel González, a young, but pregnant, single Puerto Rican woman was traveling aboard the S.S. Philadelphia when the new immigration guidelines took effect and she was detained at Ellis Island as an "alien" and "burden" to the state.

Meanwhile, on August 30 of that year, Federico Degetau, unaware of the Gonzalez situation, wrote to the U. S. Secretary of State in protest of the new rules subjecting Puerto Ricans to immigration laws.

[8] Degetau saw in the Isabel González case, the perfect "test case" for challenging the new immigration guidelines because now it would not be about whether immigration inspectors, following guidelines suffused with concepts of race and gender, deemed Isabel Gonzalez and her family desirable, but about settling the status of all the native islanders living in Puerto Rico at the time it was annexed by the United States four years earlier.

[7] The groundbreaking case, which became known as Gonzales v. Williams, was argued in the U.S. Supreme Court on December 4 and 7, 1903 and was presided by Chief Justice Melville Weston Fuller.

[14] The failure of the Degetau trust to achieve its objective was caused in part because the U.S. institution known as a "foundation" did not exist in the Puerto Rican civil code.

[16] In 2010, the legislature of the Commonwealth of Puerto Rico passed a law designating Degetau's home and farm in Aibonito a historic site and ordered the preservation of the house known as Quinta Rosacruz.