He was born in the town of Allumiere, in the Province of Lazio, then part of the Papal States, the son of Isidore Mertel, a baker from Bavaria. As a boy he studied at the local parish school, operated by the Capuchin friars in Tolfa. He then studied at the seminary in Montefiascone. After he completed his study of the humanities there, he attended the Sapienza University of Rome, where he was granted a doctorate in both civil and canon law on 16 July 1828. Mertel became a lawyer of the Roman Curia in 1831, where he was quickly promoted to the position of judge, then to Auditor of the Papal Treasury. He rose his way through the ranks of the Curia. Among his posts was that of Prefect of the Congregation of St. Ives, a society of lawyers and procurators, providing pro bono defense of the poor in the courts. Pope Pius IX named him a Cardinal Deacon on 15 March 1858, with his titular church being that of the Basilica of Sant'Eustachio. Two months later, on 16 May, Pope Pius ordained him a deacon, so that at the moment when he was created cardinal he in fact was still not in holy orders - the last layman in history to be elevated to the Cardinal’s dignity. Mertel was never ordained a priest, and at the time of his death was the last cardinal never to have been ordained a priest. One result of this was that he was the only negotiator with the Kingdom of Italy who was not able to preside at Mass but had to rely on a subordinate in priestly orders to participate in the Mass. Mertel participated in the conclave of 1878, which elected Pope Leo XIII. During the coronation ceremonies, Mertel served as protodeacon and crowned the new pope, since the Cardinal Protodeacon, Prospero Caterini, was unable to do so due to illness. In 1881 he became the Cardinal Protodeacon upon the death of Cardinal Caterini and he also chose to have his title changed to that of the Church of Santa Maria in Via Lata formerly held by the late cardinal. Pope Leo named Mertel the Vice-Chancellor of the Holy Roman Church in 1884, a post which he held until his death. At that time he had his title changed, this time to that of the Basilica of San Lorenzo in Damaso, attached to the former Chancellery of the Papal States and held by tradition by the cardinals who held that office. Because Cardinal Mertel was not an ordained priest, the cardinal-priest title of San Lorenzo in Damaso was treated as a deaconry pro illa vice. In his final years, Mertel retired to his hometown, where he died in 1899. His funeral vigil was held in the :it:Chiesa Parrocchiale Camerale Santa Maria Assunta in Cielo|Church of the Assumption, the main church of the town, and then he was buried in his family's tomb in the Shrine of the Madonna delle Grazie al Monte in the town. In 1917, eighteen years after Cardinal Mertel's death, Pope Benedict XV decreed through Canon 232 of the 1917 Code of Canon Law that all cardinals must be ordained priests.
