San Diego was appointed the following year as parochial vicar of the present-day National Shrine of Our Lady of Guadalupe in Guadalupe Nuevo, Makati, after which in 1971, he succeeded as parish priest. He worked for the Metropolitan Matrimonial Tribunal of Manila from 1964 to 1983. From 1977 to 1979, he was vicar forane of the Our Lady of Guadalupe Vicariate. He obtained his Licentiate in Canon Law, cum laude, in 1979 from the Royal and Pontifical University of Santo Tomas, with his thesis entitled Those Who Practice Psychosexual Anomaly Cannot Contract A Valid Marriage. He spent some years teaching at San Carlos Seminary in Makati and Our Lady of the Angels Seminary in Novaliches, Quezon City, and was appointed Episcopal Vicar of the Ecclesiastical District of Makati. San Diego was named parish priest of San Felipe Neri Parish in Mandaluyong in 1981. That same year, he was named Monsignor with the dignity of Honorary Prelate by the late Pope John Paul II.
The late Apostolic Nuncio to the Philippines, Archbishop Bruno Torpigliani consecrated him as bishop August 10, 1983. He served as Coadjutor Apostolic Vicar of Palawan from 1983 to 1987, and apostolic vicar from 1987 to 1995. From 1983 to 1985, he served as pastor of St. JosephParish in Narra, Palawan. He too served as rector of Seminario de San Jose in Puerto Princesa City from 1985 to 1988.
He succeeded Pedro Bantigue of Laguna in 1995, becoming the second bishop of the Diocese of San Pablo. While serving the local church of Laguna, he built a new Bishop's Residence and Chancery Office in San Pablo City. Pope John Paul II appointed San Diego as first bishop of Pasig in 2003.
On December 21, 2010, Pope Benedict XVI finally accepted San Diego's retirement as Bishop of Pasig, in compliance with Canon 401, Paragraph 1 of the 1983 Code of Canon Law requiring bishops to submit their resignation upon reaching the mandatory age of 75. Cardinal Gaudencio Rosales, concurrently Archbishop of Manila, was named Apostolic Administrator of the diocese. The Pope named Mylo Hubert Claudio Vergara, bishop of San Jose in Nueva Ecija and a native of Pasig, as new bishop of Pasig, April 20, 2011. He died on August 26, 2015, due to cardiac arrest, at age 79.[2]
