[1] Mariano served as Secretary of the Department of Agrarian Reform under the administration of President Rodrigo Duterte, who appointed him, after being nominated by the National Democratic Front.

During his term, he pressed for the passage of the Genuine Agrarian Reform Bill which aimed to distribute land to farmers for free.

He was one of five progressive legislators who were detained for 2 months in 2006 in the Batasang Pambansa Complex on charges of plotting to overthrow the government of then President Gloria Arroyo.

[4] Following his victory in the 2016 elections, President Rodrigo Duterte offered cabinet positions to the National Democratic Front of the Philippines.

The NDFP nominated Mariano for the position of Secretary of Agrarian Reform[6] which Duterte accepted at the start of his term.