[citation needed] Already noticed for his keen intelligence at an early age, Agoncillo later enrolled at the Ateneo Municipal de Manila where he was an honor student who earned high marks.

Subsequently, he transferred to the Universidad de Santo Tomás where he graduated with a Bachelor of Laws in 1879 summa cum laude.

[6] Forewarned by the plans of the governor-general, he sailed directly to Yokohama, Japan but briefly stayed and went to Hong Kong where he joined other Filipino exiles who found asylum when the revolution broke out in 1896.

Agoncillo and Jose "Sixto" Lopez were sent to Washington, D.C., United States[9] to lobby foreign entities that Filipinos are well civilized people and capable of maintaining stable government[5] and to secure recognition of Philippine independence.

He described the American system as the model which the Philippine people will follow when they are independent, and asserted that U.S. emissaries had pledged support for Filipino self-rule.

Ignoring the assertion of previous American commitments, McKinley rejected Agoncillo's request for Filipino representation at the peace talks between the U.S. and Spain and invited him to give the U.S. State Department a memorandum summarizing his views.

[11] He presented a formal protest which was called Memorial to the Senate to the president and delegates of the Spanish-American Commission saying: If the Spaniards have not been able to transfer to the Americans the rights which they did not possess; if the latter have not militarily conquered positions in the Philippines; if the occupation of Manila was a resultant fact, prepared by the Filipinos; if the international officials and representatives of the Republic of the United States of America offered to recognize the independence and sovereignty of the Philippines, solicited and accepted their alliance, how can they now constitute themselves as arbiters of the control, administration and future government of the Philippine Islands?

At that time, many Americans were also against the treaty, so they established the Anti-Imperialist League which opposed making the Philippines a colony of the United States.

De Agoncillo was appointed as Secretary of Interior in 1923 during the administration of Governor General Leonard Wood and fought for the Filipinazation of the government service.

[citation needed] While in Taal, Agoncillo continued his legal services and gave charity to poor and oppressed Filipinos.

[16] His remains were initially buried at La Loma Cemetery but was later transferred to Santuario del Santo Cristo in San Juan.