He initially studied at the Ateneo Municipal de Manila, and his contemporaries there included José Rizal, Juan Araneta, Cayetano Arellano, and Apolinario Mabini, among others.

[3] When Aguinaldo accepted the offer, Alejandrino proceeded to Hong Kong, where he helped organise the Consejo Revolucionario along with Felipe Agoncillo, José Maria Basa and Mariano Ponce.

Later, he became part of the group in the Hong Kong Committee, which included Agoncillo and Galicano Apacible, who was the head of La solidaridad, which advocated Philippines independence, as opposed to the circle led by Basa and Doroteo Cortes, who were for annexation by the United States.

Later, Aguinaldo designated Alejandrino chief of the engineers of the Army, and he directed the building of trenches in several areas, including Bulacan and Caloocan.

[3] Alejandrino and his friend, General Antonio Luna, suggested to President Emilio Aguinaldo that they build a defensive line from Novaliches to Caloocan to delay the northward advance of American troops intent on capturing the railway.