Born in Buenos Aires on 16 March 1956, Montemayor was ordained as a priest on 15 November 1985. After earning a degree in canon law he entered the diplomatic service of the Holy See on 1 July 1991 and worked in Ethiopia, Brazil, and Thailand.
On 19 June 2008 he was appointed the titular archbishop of Illici and Apostolic Nuncio to Senegal and Cabo Verde, as well as Apostolic Delegate to Mauritania.[2] On 17 September of that year he was also appointed Apostolic Nuncio to Guinea-Bissau.[3]
He received his episcopal consecration on 6 August 2008 in Buenos Aires from Archbishop Dominique Mamberti.[1]
On 22 June 2015, Pope Francis named him Apostolic Nuncio to the Democratic Republic of the Congo.[4]
On 27 September 2018, Pope Francis named him Apostolic Nuncio to Colombia.[5]
On 25 February 2023, he was named Apostolic Nuncio to Ireland.[6]
He stated in 2024 that the Church in Ireland would undergo a reorganisation, "In due time, and following careful assessment and consultation, the present Dioceses of Tuam and Killala on the one hand, and Elphin and Achonry on the other, may be governed by one Bishop in each case, just as the Dioceses of Galway and Clonfert are today governed by one Bishop. If this process evolves still further, the associated Dioceses may then merge fully under their Bishop, and, in this way, the six Dioceses in the Province of Tuam will eventually become three".[7]
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