Since he could not afford his room and board, as payment, he helped his landlady prepare food for breakfast while peddling her homemade cigars after school in the mornings.

During rainy days, he wore wooden clogs and only upon reaching school he would wear his leather shoes which he carried wrapped in paper.

After the restoration of peace following the Filipino-American War, he served as a private secretary to the Filipino civil governor of Morong Province with headquarters in Antipolo.

He analyzed the political situations for La Democracia, the Federal Party's official publication, of which he was the editor for a long time.

The paper exposed the abuses committed by the military officers against the citizens of Cavite in the concentration camp in Bacoor.

In 1904, while he was in the United States as a member of the Honorary Commission to the St. Louis Exposition he published in an American journal the independence aspiration of the Filipinos, realizing the inadvisability of the statehood plan.

Sumulong was "an effective public speaker with a high reputation for intellectual capacity and integrity" according to Claro M. Recto Jr., but he lost his senatorial bid in 1922 because of an alleged defect in the party platform.

In 1925, he was elected finally to a six-year term as senator for the Fourth Senatorial District, composed of Manila, Rizal, Laguna and Bataan.

He believed that political representation was imbalanced and that the coalition would lead to an oligarchy and to the development of a revolutionary opposition.

Sumulong, who long before Quezon adopted the slogan of "social justice", broke up with the latter and continued keeping alive an opposition.

Several hours before his death he told Jorge Bocobo and Jose Fabella that he and his party would not join in the formation of a Japanese–sponsored government.

They had 11 children, four of whom died, the seven surviving being Lumen, Demetria, Lorenzo, Paz, Juan S. Sumulong Jr., Belen and Francisco.