Born in Manila, Jacinto was proficient both in Spanish and Tagalog. He attended San Juan de Letran College, and later transferred to the University of Santo Tomas to study law. He had Manuel Quezon, Sergio Osmeña and Juan Sumulong as classmates. He did not finish college and, at the age of 19, he joined the secret society called Katipunan. He became the advisor on fiscal matters and secretary to Andrés Bonifacio. He was later known as Utak ng Katipunan.