Díaz was born in Paoay, Ilocos Norte. He was a member of La Liga Filipina, which José Rizal founded to peacefully promote reforms in the Spanish colonial administration of the Philippines. Being a resident of Tondo, Manila, he joined the La Liga council that was headed by Andrés Bonifacio, one of the founders of the Katipunan.
Díaz was one of the signatories of the Pact of Biak-na-Bato[4] in 1897. He joined the revolutionaries exiled in Hong Kong as one of the conditions set forth by the Pact.