Bishop Mercado was born on 6 June 1951 in Sta. Cruz, Manila. He was ordained to the priesthood on 19 March 1977. He was Parochial Vicar of San Isidro Parish in Pasay (1977); Spiritual Director of San Pablo Regional Seminary in Baguio City (1977-79); Director of the Pre-College Department (1979-81) and Professor and Assistant Spiritual Director (1985-88) of San Carlos Seminary in Makati City. He served as Rector of the Holy Apostles Senior Seminary in Makati City (1988-94) and Secretary of the Episcopal Commission on Seminaries under Gaudencio Cardinal Rosales (1988-94). He was Rector of the Pontificio Collegio Filipino in Rome from 1994 to 1997.
Bishop Mercado was appointed Auxiliary Bishop of Manila on 25 February 1997 and was ordained Bishop on 31 March 1997 at the Immaculate Conception Cathedral, Manila. He was appointed as the first bishop of the Diocese of Parañaque on 7 December 2002 and formally installed on 28 January 2003. He was chosen by the Catholic Bishops Conference of the Philippines as the Chairman of the Episcopal Commission on the Laity in 2009 to present. Taken from Luke 22:32, the motto in the bishop’s coat of arms is confirma fratres tuos (strengthen your brethren).