Previously, he had also served as vice mayor of Quezon City from 1968 to 1971, secretary to the commissioner of the General Authority Office from 1972 to 1981, vice governor of the Metro Manila Commission from 1979 to 1986, an assemblyman representing Quezon City in the Regular Batasang Pambansa from 1984 to 1986, representative for the city's 4th district from 1987 to 1992, director of the Metropolitan Waterworks and Sewerage System from 1979 to 1987, and chairman of the Metro Manila Authority (now Metropolitan Manila Development Authority) from 1993 to 1994.

[3][4] Mathay graduated in 1953 with bachelor's degree in Business Administration Major in Economics in the University of the Philippines, where he joined the prestigious Upsilon Sigma Phi.

In 1972, he was appointed secretary to the commissioner of the watchdog General Authority Office, a genuine recognition for his talent and integrity.

Mathay married his long-time partner Vilma Valera early in 2013, one year after the death of his first wife Sonya Gandionco.

[11] He was portrayed by Marlon Mance and Billy Jake Cortez as the young Mel Mathay in the 2023 film Martyr or Murderer.