[2] In November 1885 Matienzo Cintrón was accused by the colonial government of being a Freemason, which was illegal as it was opposed by the state Catholic Church.

Matienzo Cintrón was named to the commission which, along with Luis Muñoz Rivera, José Gómez Brioso and Federico Degetau, traveled to Spain to make official the pact with the Spanish Liberal Fusionist Party.

[2] On February 12, 1897, the Puerto Rican Autonomist Party held an assembly in San Juan, where new suggestions to the pact made by Matienzo Cintrón were approved.

Miles and his men were officially greeted that following August by a committee headed by Matienzo Cintrón, who provided the general with a banquet in his honor.

Together with Luis Muñoz Rivera (who had returned from a self exile in New York City), Antonio R. Barceló, Eduardo Georgetti, and José de Diego, he founded the Union of Puerto Rico Party.

Their political ideology was based on repeal of the Foraker Act, and the enhancement of Puerto Rican autonomy, as a pathway to full independence.

On February 8, 1912, together with Luis Lloréns Torres, Manuel Zeno Gandía and others, he wrote a manifesto demanding the independence of Puerto Rico from the United States.

[2] The government of Puerto Rico honored his memory by naming an elementary school in the town of Sabana Grande, and a plaza in Luquillo, after him.