Cardinal Herranz was born in Baena (Spain) on 31 March 1930. He joined Opus Dei in 1949 at the age of 19, while he was studying medicine in Madrid.
He has doctorates in canon law and medicine.
He was ordained on 7 August 1955 in Madrid and taught at the University of Navarre. He was actively involved in the work of the Second Vatican Council.
He was appointed undersecretary of the Pontifical Council for the Interpretation of the Code of Canon Law, and later secretary.
He was made a bishop of the titular diocese of Vertara on 15 December 1990 and consecrated on 6 January 1991 by Pope John Paul II.
On 3 December 1999, he was promoted to President of the Pontifical Council for the Interpretation of Legislative Texts and promoted to the rank of Archbishop.
Pope John Paul II elevated him to the College of Cardinals on 21 October 2003.