He was born in Bulacan, Bulacan to Juan and Mamerta de la Cruz, who changed their family name to Arellano when the Spanish colonial government decreed in 1849 that Filipinos should change their names with ones picked from a Madrid directory. Arellano studied bookkeeping at the Ateneo Municipal de Manila (now Ateneo de Manila University) and later worked as assistant clerk at the arsenal of the colonial army in Manila. He was married to Hilaria del Pilar, a sister of Marcelo H. del Pilar.
As a Revolutionary
Along with del Pilar, Arellano was an active freemason. He also solicited funds for Filipino expatriates in Spain when del Pilar had to flee to Spain because of allegedly subversive articles that appeared in the newspaper, Diariong Tagalog (Tagalog Newspaper), published by Marcelo H. del Pilar.
In 1892, Arellano and other freemasons, like Andres Bonifacio, joined La Liga Filipina, which was founded by Jose Rizal upon his return from Europe. Arellano was elected as secretary of La Liga but Rizal was deported to Dapitan in Mindanao a few days later. On July 7, 1892, when the government announced Rizal's deportation, Arellano, Bonifacio, Ladislao Diwa, Teodoro Plata, Valentin Diaz and Jose Dizon founded the Katipunan. He was elected the first president of its supreme council.
When the revolution broke out in August 1896, Deodato repaired to Bulacan and joined the brigade of Gregorio del Pilar. He became the brigade's commissary officer and fought in the battles in Bulacan during the Philippine-American War. Unfortunately, he had tuberculosis in the course of the war and died of the said disease while he and his fellow revolutionists were fighting in the Cordillera Mountains. His comrades buried him in the town cemetery of La Trinidad, Benguet.
- Quirino, Carlos. Who's who in Philippine History. Manila: Tahanan Books, 1995.
- National Historical Institute, Filipinos in History 5 vols. (Manila: National Historical Institute, 1995).